From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 934AB563FBC for ; Wed, 14 Oct 2026 18:44:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792003464; cv=none; b=UrYYEMzc2PE0GZv451nGll662NhrN6nn7U5bnfKfgohn3452GQoMCQWEN3KyTK/tfXqcWeZ6WghL5UTyw6lnByFgja1ekTZzFrT/LrcRhcnx3PIE19VVTBuk53gkVm2XSWh6jRt1c6+LbN4hqzg0945hWakb+9u0OAQxN0MCEoo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1792003464; c=relaxed/simple; bh=DezYMklGgpl75BXRVAfX62lQtl37CRtiVWlQs8CHbTY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=uhpD4OPS8fc2Jcr5rjXwBRMkZ22J19AFvCyVUWU8qacp6vTvFs6pubuaSM3qw8YdIgHyL6GDXjrGLbNXzMmXvkAobUJrl4CP14H/9rTmvY+tmLXuDLOuwrmDh56+E0ycQsq1fIhIPjPcnr3B/e2emTK98uJI/TZNgdYvDgSd3iE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kjAPCYaW; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kjAPCYaW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 699221F000C4; Wed, 14 Oct 2026 18:44:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792003463; bh=g6JScfSsWgesg/1WtK9RTmCPMl1xA6BFFNt/w568P2w=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kjAPCYaWrnOUgSCE1ZkOd4MPnsmhx/r5WxajxxuYFxUwucAaSttKAE/IXJCG+DuP2 i/SbALVGceqTj/O05V3UuqRooghqrq8qL+BxPEdjUQKfWV77/MLHM+3iZNY5Je4i6e 1TiNwuVqtJ+3R9EX8yfrLMrClw09i73W3EqwQRhBgPVNPYI0J0B2m/2nDvAPcfFy0u P+6h5mWmutCGtz1tL/lOJYTp5SP+vlc6SFLRrjtGyBnZhaTM2t4str2ceBkB2C+a+N DKe5k5FXge8ZMCjl6JOlheL3z1BqQXPRDCzup1q0YeajxmoC1X6unntHCovU3s4/m9 7xi8UByzUQgUw== Message-ID: <08a0907f-8e3a-4071-a2e4-fd1411c446f3@kernel.org> Date: Wed, 14 Oct 2026 20:44:16 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: memory: fix truncation removing mapped folios To: Pedro Falcato Cc: Baolin Wang , akpm@linux-foundation.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, qi.zheng@linux.dev, jack@suse.cz, ayushr@modal.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com> <8245424a-f068-4be5-9e58-713cbff1f8ae@kernel.org> <405811f5-ee36-4b3c-80d0-7dc6827ab5d4@linux.alibaba.com> <6660d3bb-3485-4c34-8734-3ea99df920f1@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/14/26 16:05, Pedro Falcato wrote: > On Wed, Oct 14, 2026 at 03:25:33PM +0200, David Hildenbrand (Arm) wrote: >> On 10/13/26 11:36, Baolin Wang wrote: >>> >>> >>> >>> I had considered this approach before, but it is not feasible. IIUC, the >>> sync_with_folio_pmd_zap() is used to address the race with zap_huge_pmd(), and >>> both the PMD clear and the folio mapcount decrement are performed under the PMD >>> lock in zap_huge_pmd(). >> >> Just to be clear, I am thinking about something like the following on the >> zap_pmd_range() side: >> >> diff --git a/mm/internal.h b/mm/internal.h >> index 38b1165212c94..a8820ad7e2f21 100644 >> --- a/mm/internal.h >> +++ b/mm/internal.h >> @@ -584,14 +584,15 @@ void free_pgtables(struct mmu_gather *tlb, struct >> unmap_desc *desc); >> void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte); >> >> /** >> - * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD >> + * sync_with_folio_pmd_zap - sync with concurrent zapping of folios >> * @mm: The mm_struct. >> * @pmdp: Pointer to the pmd that was found to be pmd_none(). >> * >> * When we find a pmd_none() while unmapping a folio without holding the PTL, >> - * zap_huge_pmd() may have cleared the PMD but not yet modified the folio to >> - * indicate that it's unmapped. Skipping the PMD without synchronization could >> - * make folio unmapping code assume that unmapping failed. >> + * zap_huge_pmd() or zap_empty_pte_table() may have cleared the PMD but not >> + * yet modified the mapped folios to indicate they are unmapped. Skipping the >> + * PMD without synchronization could make folio unmapping code assume that >> + * unmapping failed. >> * >> * Wait for concurrent zapping to complete by grabbing the PTL. >> */ >> diff --git a/mm/memory.c b/mm/memory.c >> index 8b0c2c735d3de..c585f5eb255c9 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2139,9 +2139,7 @@ static inline unsigned long zap_pmd_range(struct >> mmu_gather *tlb, >> continue; >> } >> /* fall through */ >> - } else if (details && details->single_folio && >> - folio_test_pmd_mappable(details->single_folio) && >> - next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) { >> + } else if (details && details->single_folio && pmd_none(*pmd)) { >> sync_with_folio_pmd_zap(tlb->mm, pmd); >> } >> if (pmd_none(*pmd)) { >> >>> >>> However, the race here is different: tlb_flush_rmaps() is under the PTE lock, >>> while zap_empty_pte_table() is under the PMD lock, and reordering between the >>> two functions is also possible. That is, the folio mapcount operation could >>> execute after the acquire barrier (spin_lock()) of zap_empty_pte_table(), >>> allowing a reader on CPU 0 to incorrectly observe that the PMD is none while >>> mapcount > 0. >> >> I think we really just have to make sure to flush rmaps before zapping the PMD. >> Then it's really just the same thing we had before with sync_with_folio_pmd_zap(). > > Hmm, could we viably replace sync_with_folio_pmd_zap() with the memory barriers > instead? Seems lighter-weight and functionally equivalent? For the ordinary PMD zap case it's really: 1) Clear PMD 2) Drop refcount+mapcount So I don't think it's a direct replacement. If we had a proper way to invalidate + adjust + clear it could work. Like 1) Invalidate 2) Drop refcount+mapcount 3) Clear But in general I think: a) Likely all places where we have existing sync_with_folio_pmd_zap() have this problem (2). b) I don't want us to throw around memory barriers that way. This all belongs into a helper (like sync_with_folio_pmd_zap()) which we only call when required (e.g., details && details->single_folio) and that properly documents all the nasty details. c) Maybe there is some smart way to decide whether we need a memory barrier or a proper lock for now inside sync_with_folio_pmd_zap(). Based on the folio size should do (PMD vs. < PMD). But I assume grabbing the PMD lock briefly is not really a concern: if there would be something mapped we would have to lock it, and if it would be a PTE table we would lock that instead. [insert rant about our PMD table locking] -- Cheers, David