From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794489413; b=MjQ3jYCSBaOUN1mFLBWnJnrHUm3oxzeGvcvHmmQZbnL/Ey1wZYsSgFbgc6SPZOU92P3G oGoRlcXzpiyYs2Q9yom1/uxOMTGbxEdta8zC0zOOGHHvA9EJiZ0fLO40HPtx61RDEUhXL FwZbRnJL5jnkztnxSBhLimgqC7hWr+oFumzL6zdokF3Kac4uGxpoMkEvsqdiWqDz2yMfX Lv5mCH2e4GhGVAb/QrPOPNZkqWXqEmkWX6CPOvOIGo5XmOqSudVcfWXO5FXjeaEdCK3zz iKrvhpd9Rl4yxMh6GW0dbCNb2eqDf5lLHfbipSRE5lSm9EyFdlJdRyyA52QkyznhF+A== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794489413; h=Received:DKIM-Signature:Message-ID:Date:MIME-Version:User-Agent: Subject:To:Cc:References:From:Content-Language:Autocrypt:In-Reply-To: Content-Type:Content-Transfer-Encoding; bh=i2Oes8RZDkSEXFI8H+2vd0Da2cFliXVxwNXn1JhwifA=; b=R6CpjBXbo9UkPQwauKY841zadvBR08T6flCTcsHMJnIuDrGCX8TJ68dYJISl1CRYRiF3 rlHKnZEYJbEMy/C6I1D+NP2M8vkzTnpw0gUxnj2ulyrBCDEAguw7IAUqtNVqZhRmH/lvn JILmt45F5oZZmsJWgnZ9pj5cveADbENyimM+C1ScjwX0eUqUHjZYafGt6mVRAiG87DCaj JYHMFbRSLGeWYyeb9RngjgHgU3qPnZqyk5Q9HYk9L42BEy4ZmzpD1o4DyTA4L1S6GuwXU EeRMVqq+qyi/Q+e7BXgLkjFDgHk1XZRi8iLbt5/r/0KgPbdHp4HtqGDFwjOkUNec5HA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="OWHHd/ls"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 72201606A323; Thu, 12 Nov 2026 13:16:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=OWHHd/ls Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA5611F00A05; Thu, 12 Nov 2026 13:16:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794489413; bh=i2Oes8RZDkSEXFI8H+2vd0Da2cFliXVxwNXn1JhwifA=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=OWHHd/lsZYkX+Jq2T55F046KvGJkwVb6ZNJHRXrnIltU3y9ehCady9llvQYz7eT6C b6gwiEygyCndpaJIRyg+GavO8iI+YCfZ2Uwp35NT0l9voskGAjqZjcNzrW7ElwS9vq xA2xR8/J9T5ZBK4HMLhcLMY5URFYgvGfGjWh0b1z5FjYipbPVvuLhazpnlXmjoISp0 28UuXxrC6BUTP/lTG8Y9aJkxGNLyQrFX3s04RL5B3nRopwvnht2pma59Ed4xyBMoWC GkpVbhIoxBJV+NY5ZH8W+H4wvUY/uz5MDTIPd+UxtdLDO+3fcrB0mm6Vh6AdesMpbH e/KSk7ObVX1PA== Message-ID: <86206f64-fefa-4a13-95bc-eddf756eecc8@kernel.org> Date: Thu, 12 Nov 2026 14:16:33 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/9] mm: limit pagecache folio refcount when mapping pages To: Pedro Falcato Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Miklos Szeredi , Alexander Viro , Christian Brauner , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Arnd Bergmann , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard , "Jose A. Perez de Azpillaga" , Josef Bacik , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Neal Cardwell , Kuniyuki Iwashima , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, linux-arch@vger.kernel.org, iommu@lists.linux.dev, netdev@vger.kernel.org, Chris Mason , stable@vger.kernel.org References: <20261110-refcount_overflow_stable-v3-0-318625d63a98@kernel.org> <20261110-refcount_overflow_stable-v3-1-318625d63a98@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit > Mostly LGTM, with some comments: > > Reviewed-by: Pedro Falcato Thanks for the review, Pedro! > >> --- >> include/linux/mm.h | 35 +++++++++++++++++++++++++++++++---- >> mm/filemap.c | 6 ++++++ >> mm/huge_memory.c | 29 +++++++++++++++++++++++++++-- >> mm/memory.c | 33 +++++++++++++++++++++++++++++---- >> mm/migrate.c | 11 +++++++++++ >> mm/userfaultfd.c | 4 ++++ >> mm/util.c | 18 ++++++++++++++++++ >> 7 files changed, 126 insertions(+), 10 deletions(-) >> >> diff --git a/include/linux/mm.h b/include/linux/mm.h >> index e6da76fbe8e83..6c7c29a086bd1 100644 >> --- a/include/linux/mm.h >> +++ b/include/linux/mm.h >> @@ -2229,9 +2229,36 @@ vm_fault_t finish_fault(struct vm_fault *vmf); >> * back into memory. >> */ >> >> -/* 127: arbitrary random number, small enough to assemble well */ >> -#define folio_ref_zero_or_close_to_overflow(folio) \ >> - ((unsigned int) folio_ref_count(folio) + 127u <= 127u) >> +/* Leave separate reserves for administrators and concurrent reference gains. */ >> +#define FOLIO_REF_COUNT_LIMIT (INT_MAX - (1U << 26)) >> +#define FOLIO_REF_COUNT_ADMIN_LIMIT (INT_MAX - (1U << 25)) >> + >> +bool __folio_ref_count_over_limit(unsigned int ref_count); >> + >> +/** >> + * folio_ref_count_over_limit - Check whether to refuse more folio references. >> + * @folio: The folio. >> + * >> + * Check before adding page table mappings or other significant refcount users >> + * to pagecache folios (!hugetlb), either before acquiring references or >> + * afterwards with rollback if the operation is refused. >> + * >> + * Checking other folios does no harm. >> + * >> + * This check is an approximation; we leave some headroom such that concurrent >> + * increments, batches, temporary references, or other bounded refcount >> + * consumers are not a problem. >> + * >> + * Return: True if the caller should refuse the operation, false otherwise. >> + */ >> +static inline bool folio_ref_count_over_limit(const struct folio *folio) >> +{ >> + const unsigned int ref_count = folio_ref_count(folio); >> + >> + if (likely(ref_count < FOLIO_REF_COUNT_LIMIT)) >> + return false; >> + return __folio_ref_count_over_limit(ref_count); >> +} >> >> /** >> * folio_get - Increment the reference count on a folio. >> @@ -2243,7 +2270,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf); >> */ >> static inline void folio_get(struct folio *folio) >> { >> - VM_BUG_ON_FOLIO(folio_ref_zero_or_close_to_overflow(folio), folio); >> + VM_WARN_ON_FOLIO(folio_ref_count(folio) <= 0, folio); >> folio_ref_inc(folio); >> } >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index 9871b4f32a916..0f6d57432a7e5 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -3968,6 +3968,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, >> unsigned long end; >> vm_fault_t map_ret; >> >> + if (unlikely(folio_ref_count_over_limit(folio))) { >> + folio_ref_dec(folio); >> + goto next; >> + } >> + > > It's a bit odd that you take a ref, only to check later if we're over the > limit, and ditch the folio. Could you move this to next_uptodate_folio()? That might be even better (and avoids one conflict with Andrew's tree). Let me take a look. > >> addr += (xas.xa_index - last_pgoff) << PAGE_SHIFT; >> vmf->pte += xas.xa_index - last_pgoff; >> last_pgoff = xas.xa_index; >> @@ -4009,6 +4014,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf, >> mmap_miss - 1); >> } >> >> +next: >> folio_unlock(folio); >> } while ((folio = next_uptodate_folio(&xas, mapping, end_pgoff)) != NULL); >> add_mm_counter(vma->vm_mm, folio_type, rss); >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 4608540e40d46..439cfe446cea4 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -1626,6 +1626,7 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr, >> { >> struct mm_struct *mm = vma->vm_mm; >> pgtable_t pgtable = NULL; >> + vm_fault_t ret = VM_FAULT_NOPAGE; >> spinlock_t *ptl; >> pmd_t entry; >> >> @@ -1662,6 +1663,10 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr, >> if (is_huge_zero_folio(fop.folio)) { >> entry = pmd_mkspecial(entry); >> } else { >> + if (unlikely(folio_ref_count_over_limit(fop.folio))) { >> + ret = VM_FAULT_SIGBUS; >> + goto out_unlock; >> + } >> folio_get(fop.folio); > > I see you use this pattern often. Would it make sense to have a > bool folio_get_unless_overflow()? I commented somewhere (in v2 I think) that I played with using folio_get_checked() from #2, but didn't quite like it so I discarded it as (a) it cannot be used everywhere here, as I'd need more functions to add multiple references. next_uptodate_folio() also could use it as I would need yet another temporary helper for a try_get(). (b) these changes here are going to get reverted fairly soon once I change the way how mappings are refcounted (which is, unfortunately, not suitable for stable, otherwise I wouldn't be throwing in this temporary change in the first place) So everything that is expected to stay in later patches uses the proper helper. [...] >> >> + if (unlikely(folio_ref_count_over_limit(folio))) { >> + /* Skip this PUD; it can be faulted in later. */ >> + ret = (dst_vma->vm_flags & VM_MIXEDMAP) ? -ENOMEM : 0; > > A comment as to what's going on here with VM_MIXEDMAP would be useful. I guess > the idea is that faulting on a mixedmap later will not necessarily hand you the > exact same page fault result? I can throw in a comment, yes. [...] >> +bool __folio_ref_count_over_limit(unsigned int ref_count) >> +{ >> + if (ref_count >= FOLIO_REF_COUNT_ADMIN_LIMIT) >> + return true; >> + /* Background threads must not consume admin reserves. */ >> + if (current->flags & PF_KTHREAD) >> + return true; >> + /* Neither must interrupt contexts on behalf of the interrupted task. */ >> + if (!in_task()) >> + return true; >> + /* Capability checks are not safe with IRQs disabled. */ >> + if (irqs_disabled()) >> + return true; >> + /* Use the task's credentials, which filesystem overrides cannot change. */ >> + return !has_capability_noaudit(current, CAP_SYS_RESOURCE); >> +} > > In case this function ever returns over limit, we probably want to log this > in the kernel log. If this function gets the folio you could even print > the problematic file offset/inode/filesystem/etc. Ratelimited of course. You mean, if this function returns "true", right? Is printing always fine? (e.g., irqs disabled?) > >> +EXPORT_SYMBOL_GPL(__folio_ref_count_over_limit); > > I'm fairly sure this will break the build for a number of non-GPL drivers > when calling folio_get() with CONFIG_DEBUG_VM=y. You mean, breaking OOT drivers, right? I can switch to EXPORT_SYMBOL if that's a concern. -- Cheers, David