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 8E6882F7EEE for ; Thu, 18 Jun 2026 17:07:46 +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=1781802467; cv=none; b=KjK/xIVlx0kNkWDUoohj+Ffdy4Xcr0BhLYluOryiDU1BOJk7ojUqNmOe7xp+lJ4szYHwagBimwe/i6kpHhNXE+XpD/8w/Dk7zpHTvWNYLVY3K8d1FQ0jY5tC+XIIkW0yvTu/GdFgNcu+XbUz89Nw1IJYUD5RWzkcMwGZDRdbwbc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781802467; c=relaxed/simple; bh=+V+aqbaIrrmAEXPVbIsW9RlFFH/GqKT9PXnFphSsLwE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VrHkq1y0hZUEb8C4VU/Ejj5wQEC6lMTuafBSBSLDouynU5hmlAHsRhpT3VaaGygSu0V5C7V4kWjNWfmlNtAtfoptZx3dXZwI8mVjLmhQJn87N0V68WBmjN1BVGgcJ1Y1JHMLOxfGxjX1WlXx89o3LSaq8tu1jy+Kvt1P22AqoIU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kx8zyzO7; 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="kx8zyzO7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E918C1F000E9; Thu, 18 Jun 2026 17:07:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781802466; bh=CaAA2B6CTQIeAfF9nNT4236V48ifhECzdxsqKpDS0ls=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=kx8zyzO7PD8eRTepsHFSKPOmTVEjflYhbBq8sJgG/XnkcbumUjsrnPnpb8BAk96NG uDKM2RRHU8sthRNpOO7lceCoOvyiaPjBVH5tJcRgET75MmRsovul7z3M4BX3nppi76 klICesKuwG3QdWmmy5F1qI9UaneeNSKncQQLYVltcOdBO2JPXaX6X1qhYkCP+K3e1O x1T+0i1oTYqiEuCiX23YlS23fEhMDDyP2ytPDvMkl9c+DiUUJbDaq0tnMkRJiTiCza sMLpe3fp3/E4dNMPbVx49HvrGsvlLUHjw3bNtzS9YUFZBBc1oDW/OZqvXsnA4CEc9D K9dR/6xjecUDQ== Message-ID: <929d36a3-f08d-47e5-94c0-b06739dac74c@kernel.org> Date: Thu, 18 Jun 2026 19:07:39 +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 3/3] mm: read remote memory without the mmap lock where possible To: Usama Arif , Rik van Riel Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , Dmitry Ilvokhin , Borislav Petkov , Dave Hansen , Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan References: <20260618170157.1375279-1-usama.arif@linux.dev> 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: <20260618170157.1375279-1-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/18/26 19:01, Usama Arif wrote: > On Tue, 16 Jun 2026 15:03:00 -0400 Rik van Riel wrote: > >> __access_remote_vm() takes mmap_read_lock() for the entire transfer and >> uses get_user_pages_remote(), which faults pages in. For the common >> case of reading memory that is already resident -- /proc/PID/cmdline, >> /proc/PID/environ, ptrace PEEK of resident pages -- the mmap lock is >> unnecessary and is badly contended on large machines. >> >> Add an opportunistic, read-only fast path that transfers what it can >> without the mmap lock. For each address it takes the per-VMA lock with >> lock_vma_under_rcu(), re-checks the read-side VMA permissions, and uses >> folio_walk_start(..., FW_VMA_LOCKED) to grab a short-lived reference to >> a present page before copying it out. Anything non-trivial -- a not- >> present page (needs faulting), a hugetlb or VM_IO/VM_PFNMAP mapping, or >> a race with a VMA writer -- falls back to the existing mmap_lock path >> for the remainder. >> >> untagged_addr_remote() asserts the mmap lock, so add an unlocked variant >> for the fast path; the untag mask is a stable per-mm value. >> >> Only reads are handled here; writes keep using the slow path. >> >> Assisted-by: Claude:claude-opus-4-8 >> Signed-off-by: Rik van Riel >> --- >> arch/x86/include/asm/uaccess_64.h | 12 +++ >> include/linux/uaccess.h | 11 ++ >> mm/memory.c | 166 +++++++++++++++++++++++++++++- >> 3 files changed, 188 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h >> index 4a52497ba6a1..c6fac900a747 100644 >> --- a/arch/x86/include/asm/uaccess_64.h >> +++ b/arch/x86/include/asm/uaccess_64.h >> @@ -51,6 +51,18 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, >> (__force __typeof__(addr))__untagged_addr_remote(mm, __addr); \ >> }) >> >> +/* Same as __untagged_addr_remote(), but usable without the mmap lock held. */ >> +static inline unsigned long __untagged_addr_remote_unlocked(struct mm_struct *mm, >> + unsigned long addr) >> +{ >> + return addr & READ_ONCE((mm)->context.untag_mask); >> +} >> + >> +#define untagged_addr_remote_unlocked(mm, addr) ({ \ >> + unsigned long __addr = (__force unsigned long)(addr); \ >> + (__force __typeof__(addr))__untagged_addr_remote_unlocked(mm, __addr); \ >> +}) >> + >> #endif >> >> #define valid_user_address(x) \ >> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h >> index 8a264662b242..c8c83372c9d8 100644 >> --- a/include/linux/uaccess.h >> +++ b/include/linux/uaccess.h >> @@ -34,6 +34,17 @@ >> }) >> #endif >> >> +/* >> + * Like untagged_addr_remote(), but for callers that stabilize @mm by other >> + * means (e.g. a per-VMA lock) and must not assert the mmap lock. >> + */ >> +#ifndef untagged_addr_remote_unlocked >> +#define untagged_addr_remote_unlocked(mm, addr) ({ \ >> + (void)(mm); \ >> + untagged_addr(addr); \ >> +}) >> +#endif >> + >> #ifdef masked_user_access_begin >> #define can_do_masked_user_access() 1 >> # ifndef masked_user_write_access_begin >> diff --git a/mm/memory.c b/mm/memory.c >> index 86a973119bd4..0b23b82eaa18 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -42,6 +42,8 @@ >> #include >> #include >> #include >> +#include >> +#include >> #include >> #include >> #include >> @@ -7062,6 +7064,153 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr, >> EXPORT_SYMBOL_GPL(generic_access_phys); >> #endif >> >> +/* >> + * The fast path uses folio_walk_start(FW_VMA_LOCKED), which needs the per-VMA >> + * lock and RCU-freed page tables to walk page tables without the mmap lock. >> + */ >> +#if defined(CONFIG_PER_VMA_LOCK) && defined(CONFIG_MMU_GATHER_RCU_TABLE_FREE) >> +/* >> + * Opportunistic lockless fast path for __access_remote_vm() reads. >> + * >> + * Memory already resident in @mm can be read without taking the heavily >> + * contended mmap_lock: a per-VMA lock stabilizes the VMA, and folio_walk_start() >> + * with FW_VMA_LOCKED grabs a short-lived reference to a present page via an >> + * RCU/PTL protected page table walk (relying on MMU_GATHER_RCU_TABLE_FREE). >> + * >> + * Anything that would require faulting a page in, touching a hugetlb or >> + * VM_IO/VM_PFNMAP mapping, or that races a VMA writer is left to the mmap_lock >> + * path in __access_remote_vm(). Only reads are handled here. >> + * >> + * Returns the number of bytes transferred via the fast path. >> + */ >> +static int access_remote_vm_fast(struct mm_struct *mm, unsigned long addr, >> + void *buf, int len, unsigned int gup_flags) >> +{ >> + void *old_buf = buf; >> + >> + addr = untagged_addr_remote_unlocked(mm, addr); >> + >> + while (len) { >> + struct vm_area_struct *vma; >> + vm_flags_t vm_flags; >> + >> + vma = lock_vma_under_rcu(mm, addr); >> + if (!vma) >> + break; >> + >> + /* >> + * Mirror the read-side permission checks of check_vma_flags(), >> + * and exclude what FW_VMA_LOCKED cannot handle (hugetlb) or what >> + * needs the ->access() handler (VM_IO/VM_PFNMAP). Checked once >> + * per VMA; anything not positively allowed falls back to the >> + * slow path, which re-validates everything. >> + */ >> + vm_flags = vma->vm_flags; >> + if ((vm_flags & (VM_IO | VM_PFNMAP)) || >> + is_vm_hugetlb_page(vma) || vma_is_secretmem(vma) || >> + (!(vm_flags & VM_READ) && >> + (!(gup_flags & FOLL_FORCE) || !(vm_flags & VM_MAYREAD)))) { >> + vma_end_read(vma); >> + break; >> + } > > This should also do the FOLL_ANON check from check_vma_flags(). > > check_vma_flags() rejects non-anonymous VMAs when FOLL_ANON is set: > > if ((gup_flags & FOLL_ANON) && !vma_anon) > return -EFAULT; Duplicating GUP logic in a non-GUP file. Splendid. :) -- Cheers, David