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Tue, 17 Jun 2025 10:03:14 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id FR6GM2I9UWgFFgAAD6G6ig (envelope-from ); Tue, 17 Jun 2025 10:03:14 +0000 Date: Tue, 17 Jun 2025 12:03:13 +0200 From: Oscar Salvador To: David Hildenbrand Cc: Andrew Morton , Muchun Song , James Houghton , Peter Xu , Gavin Guo , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/5] mm,hugetlb: Document the reason to lock the folio in the faulting path Message-ID: References: <20250612134701.377855-1-osalvador@suse.de> <20250612134701.377855-3-osalvador@suse.de> <44f0f1cc-307a-46e3-9e73-8b2061e4e938@redhat.com> <1297fdd5-3de2-45bc-b146-e14061643fee@redhat.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=us-ascii Content-Disposition: inline In-Reply-To: <1297fdd5-3de2-45bc-b146-e14061643fee@redhat.com> X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MISSING_XM_UA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_SEVEN(0.00)[8]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Spam-Level: X-Spam-Flag: NO X-Rspamd-Queue-Id: 657D021190 X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Score: -4.51 On Mon, Jun 16, 2025 at 04:41:20PM +0200, David Hildenbrand wrote: > On 16.06.25 16:10, Oscar Salvador wrote: > > What do you mean by stable? > > The same "stable" you used in the doc, that I complained about ;) Touche :-D > > In the generic faulting path, we're not worried about the page going away > > because we hold a reference, so I guess the lock must be to keep content stable? > > What you want to avoid is IIRC, is someone doing a truncation/reclaim on the > folio while you are mapping it. Ok, I see. I thought it was more about holding writes, but this makes sense. > Take a look at truncate_inode_pages_range() where we do a folio_lock() > around truncate_inode_folio(). > > In other words, while you hold the folio lock (and verified that the folio > was not truncated yet: for example, that folio->mapping is still set), you > know that it cannot get truncated concurrently -- without holding other > expensive locks. > > Observe how truncate_cleanup_folio() calls > > if (folio_mapped(folio)) > unmap_mapping_folio(folio); > > To remove all page table mappings. > > So while holding the folio lock, new page table mappings are not expected to > appear (IIRC). Ah ok, so it's more that we don't end up mapping something that's not there anymore (or something completely different). > > I mean, yes, after we have mapped the page privately into the pagetables, > > we don't have business about content-integrity anymore, so given this rule, yes, > > I guess hugetlb_wp() wouldn't need the lock (for !anonymous) because we already > > have mapped it privately at that point. > > That's my understanding. And while holding the PTL it cannot get unmapped. > Whenever you temporarily drop the PTL, you have to do a pte_same() check to > make sure concurrent truncation didn't happen. Yap, hugetlb_wp() drops the locks temporarily when it needs to unmap the private page from other processes, but then does the pte_same() check. > So far my understanding at least of common filemap code. > > > > > But there's something I don't fully understand and makes me feel uneasy. > > If the lock in the generic faultin path is to keep content stable till we > > have mapped it privately, wouldn't be more correct to also hold it > > during the copy in hugetlb_wp, to kinda emulate that? > As long there us a page table mapping, it cannot get truncated. So if you > find a PTE under PTL that maps that folio, truncation could not have > happened. I see, this makes a lot of sense, thanks for walking me through David! Alright, then, with all this clear now we should: - Not take any locks on hugetlb_fault()->hugetlb_wp(), hugetlb_wp() will take it if it's an anonymous folio (re-use check) - Drop the lock in hugetlb_no_page() after we have mapped the page in the pagetables - hugetlb_wp() will take the lock IFF the folio is anonymous This will lead to something like the following: diff --git a/mm/hugetlb.c b/mm/hugetlb.c index dfa09fc3b2c6..4d48cda8a56d 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -6198,6 +6198,8 @@ static vm_fault_t hugetlb_wp(struct vm_fault *vmf) * in scenarios that used to work. As a side effect, there can still * be leaks between processes, for example, with FOLL_GET users. */ + if (folio_test_anon(old_folio)) + folio_lock(old_folio); if (folio_mapcount(old_folio) == 1 && folio_test_anon(old_folio)) { if (!PageAnonExclusive(&old_folio->page)) { folio_move_anon_rmap(old_folio, vma); @@ -6212,6 +6214,8 @@ static vm_fault_t hugetlb_wp(struct vm_fault *vmf) } VM_BUG_ON_PAGE(folio_test_anon(old_folio) && PageAnonExclusive(&old_folio->page), &old_folio->page); + if (folio_test_anon(old_folio)) + folio_unlock(old_folio); /* * If the process that created a MAP_PRIVATE mapping is about to perform @@ -6537,11 +6541,6 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, } new_pagecache_folio = true; } else { - /* - * hugetlb_wp() expects the folio to be locked in order to - * check whether we can re-use this page exclusively for us. - */ - folio_lock(folio); anon_rmap = 1; } } else { @@ -6558,7 +6557,8 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, /* Check for page in userfault range. */ if (userfaultfd_minor(vma)) { - folio_unlock(folio); + if (!anon_rmap) + folio_unlock(folio); folio_put(folio); /* See comment in userfaultfd_missing() block above */ if (!hugetlb_pte_stable(h, mm, vmf->address, vmf->pte, vmf->orig_pte)) { @@ -6604,6 +6604,13 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, new_pte = huge_pte_mkuffd_wp(new_pte); set_huge_pte_at(mm, vmf->address, vmf->pte, new_pte, huge_page_size(h)); + /* + * This folio cannot have been truncated since we were holding the lock, + * and we just mapped it into the pagetables. Drop the lock now. + */ + if (!anon_rmap) + folio_unlock(folio); + hugetlb_count_add(pages_per_huge_page(h), mm); if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) { /* Optimization, do the COW without a second fault */ @@ -6619,8 +6626,6 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, */ if (new_folio) folio_set_hugetlb_migratable(folio); - - folio_unlock(folio); out: hugetlb_vma_unlock_read(vma); @@ -6639,8 +6644,8 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, backout_unlocked: if (new_folio && !new_pagecache_folio) restore_reserve_on_error(h, vma, vmf->address, folio); - - folio_unlock(folio); + if (!anon_rmap) + folio_unlock(folio); folio_put(folio); goto out; } @@ -6805,21 +6810,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, /* Fallthrough to CoW */ } - /* - * We need to lock the folio before calling hugetlb_wp(). - * Either the folio is in the pagecache and we need to copy it over - * to another file, so it must remain stable throughout the operation, - * or the folio is anonymous and we need to lock it in order to check - * whether we can re-use it and mark it exclusive for this process. - * The timespan for the lock differs depending on the type, since - * anonymous folios only need to hold the lock while checking whether we - * can re-use it, while we need to hold it throughout the copy in case - * we are dealing with a folio from a pagecache. - * Representing this difference would be tricky with the current code, - * so just hold the lock for the duration of hugetlb_wp(). - */ folio = page_folio(pte_page(vmf.orig_pte)); - folio_lock(folio); folio_get(folio); if (flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) { @@ -6835,7 +6826,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, flags & FAULT_FLAG_WRITE)) update_mmu_cache(vma, vmf.address, vmf.pte); out_put_page: - folio_unlock(folio); folio_put(folio); out_ptl: spin_unlock(vmf.ptl); This should be patch#2 with something like "Sorting out locking" per title, and maybe explaining a bit more why the lock in hugelb_wp for anonymous folios. What do you think? -- Oscar Salvador SUSE Labs