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 57DEC72618 for ; Tue, 7 Jul 2026 12:24:57 +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=1783427098; cv=none; b=hnYZhhjuMqHb8laDRcEnY7Ii04+wYixDpIqsS06Ow2gfw3JmzrI/Ar0UnHi+G0p3QhBmyE1OypCCSjvG4ympNHXokxyUfHyT0uTPuM2nGE6gZoCeyH511NVgHvYCAeKLYMMZHj8cEEwJ9WCViVg8Hsms/5mn9+7C3/ES70F0pzU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783427098; c=relaxed/simple; bh=U2DmQDW6W/LGFxOWsb6gN8B/kidBEKg8yb8EhQBRdcA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=c4isIIaU1nbR2e5wn+ZQ/JxSbK6fHZNJFG1QxYnnn44cUkbtqlAiFiNX+xbVXPL6Z92U1yrr7FRrcSSJrVKfV6Q1e9/AjAbpDJ+B0+1m6XXK6/7MZI7fnHuhNQtFHG60cxVa4leSKf9FBmOn59gb9Ixp2L4bBeRRyp6YQqVI4t8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T6tmb7Sh; 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="T6tmb7Sh" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A4F851F00A3A; Tue, 7 Jul 2026 12:24:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783427097; bh=NfZpOMpkaJDzLOKi3MjXRffgVPNSMGoyto8kV1jnWd8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=T6tmb7Shq5+cgN69zOoULFP7X9TcvzY8I59VD0foVDxtRg/4ABK52SR5rXSXTUvf7 FGrSaIYQIlGLYK0xisZkr/jQQY03bpN2KjD7h26E1qL67liUSyDBS6rm5Kz1OpFmHS oHYcRiYa8g4bHcg/Mntj0elMmTu5xoCM37zMocd58szp3DuhvBZ1yCc8KvJDKDT5ZB 58+bCxAVhKgi8SgDfqL7VhfmHwLc4ZaPjuB9rEggMJQmv1n6IStRBi/9dbEIlKxgwg YU7iY9LjUTyBRiJO7NrDLZ3AQQjQAlFjvQRSvhLmgl7n83NbYPJTVBFMGtQsjPKuZX cOFxKkMJzD7Gw== Date: Tue, 7 Jul 2026 13:24:46 +0100 From: Lorenzo Stoakes To: 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 , David Hildenbrand , "Liam R. Howlett" , Vlastimil Babka , Suren Baghdasaryan , kernel-team@meta.com Subject: Re: [PATCH 2/3] mm/pagewalk: let folio_walk_start() run under the per-VMA lock Message-ID: References: <20260625015053.2445008-1-riel@surriel.com> <20260625015053.2445008-3-riel@surriel.com> <610d1c95a2f9131d5c018304f9a90afb33e8a032.camel@surriel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <610d1c95a2f9131d5c018304f9a90afb33e8a032.camel@surriel.com> On Thu, Jun 25, 2026 at 07:20:22AM -0400, Rik van Riel wrote: > On Thu, 2026-06-25 at 08:34 +0100, Lorenzo Stoakes wrote: > > Rik, it really would have helped if you'd replied to review :) > > > > On Wed, Jun 24, 2026 at 09:50:52PM -0400, Rik van Riel wrote: > > > folio_walk_start() asserts the mmap lock is held.  For callers that > > > only > > > need to read a single, already-present page, the mmap lock is a > > > heavy and > > > often badly contended hammer.  Such a caller can instead hold the > > > per-VMA > > > lock, which keeps the VMA itself stable. > > > > > > > > > The per-VMA lock does not, however, keep the page tables walked > > > below that > > > VMA from being freed.  A concurrent munmap() or THP collapse of an > > > adjacent region in the same mm can free a shared upper-level table, > > > and > > > > Yeah I need to update the documentation on this at > > https://docs.kernel.org/mm/process_addrs.html it's more subtle than > > written > > there. Actually not so sure I do now :) I mean maybe I need to be explicit about the need for PTL's too. I guess 'other than the PTLs your page table walker will take' is kinda implicit there. > > > > Firstly you're wrong about munmap() - it acquires the VMA lock of the > > VMAs freed > > in the range and will only remove an upper level table if the entire > > range is > > spanned. > > > > And that's the only way higher level tables can be removed. > > > > PTE page tables can be removed via MADV_DONTNEED, but that a. > > acquires the VMA > > lock and b. frees the PTE page table under RCU. > > > > A THP collapse can happen concurrently, but PTEs are freed under RCU > > so you > > don't need to do this GUP fast imitating stuff. > > > > > THP collapse (collapse_huge_page() -> retract_page_tables()) frees > > > page > > > tables of VMAs whose lock it does not hold.  Page table freeing > > > > retract_page_tables() -> pte_free_defer() -> RCU > > try_collapse_pte_mapped_thp() -> pte_free_defer() -> RCU > > One issue here is that while we can safely read > the old page table under the RCU read lock, in > the middle of a THP collapse there is no guarantee > that the old page table points at the process's > current memory. See below. > > Khugepaged could fix this in one of two ways: > - zap all readers with an IPI, and use that as > synchronization > - make sure the old page table's PTEs point at > the individual pages inside the new PMD > > Right now khugepaged does the first. This is confusing too. Khugepaged _could_ fix this one of two ways, right now it _does_ the first? You mean pmdp_get_lockless_sync() calls tlb_remove_table_sync_one()? But that's for GUP fast right, not folio_walk_start/end()? > > Relying only on the RCU read lock to read the > page table could result in us seeing old page > table contents, that no longer point at the > current process memory. > > Unless I'm missing something... I think so! Otherwise we'd already have very broken page table walkers right? So: pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, unsigned long addr, spinlock_t **ptlp) { spinlock_t *ptl; pmd_t pmdval; pte_t *pte; again: pte = __pte_offset_map(pmd, addr, &pmdval); if (unlikely(!pte)) return pte; ptl = pte_lockptr(mm, &pmdval); spin_lock(ptl); if (likely(pmd_same(pmdval, pmdp_get_lockless(pmd)))) { *ptlp = ptl; return pte; } pte_unmap_unlock(pte, ptl); goto again; } __pte_offset_map() takes the RCU lock, then locklessly retrieves a pointer to the PTE page table. As mentioned above, khugepaged retraction causing PTE page table freeing is also under RCU, so this is safe: retract_page_tables() -> pte_free_defer() -> RCU try_collapse_pte_mapped_thp() -> pte_free_defer() -> RCU And on page table retraction, importantly, the PMD entry is cleared _with the PTE PTL held_, e.g. in retract_page_tables(): ptl = pte_lockptr(mm, pmd); if (ptl != pml) spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); ... if (likely(file_backed_vma_is_retractable(vma))) { pgt_pmd = pmdp_collapse_flush(vma, addr, pmd); ... } if (ptl != pml) spin_unlock(ptl); For anon, we take the mmap write lock/VMA write lock anyway. And going back to pte_offset_map_lock() - we _recheck the PMD under the PTL_. So if the PMD was since cleared, we don't get anything stale. If it wasn't, then we hold the PTE PTL and exclude khugepaged from changing things under us. Thus folio_walk_start(), which holds the PTE PTL, is perfectly fine, and folio_walk_end() marks the point at which things are no longer safe (but then we're done with the folio). So again I don't think there's any problem here, actually. Thanks, Lorenzo