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Tue, 17 Jun 2025 12:50:40 +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 J9wSF6BkUWj+SgAAD6G6ig (envelope-from ); Tue, 17 Jun 2025 12:50:40 +0000 Date: Tue, 17 Jun 2025 14:50:38 +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: <1297fdd5-3de2-45bc-b146-e14061643fee@redhat.com> <11a1d0f7-ef4e-4836-9bde-d7651eebcd03@redhat.com> <3eb8e1e2-5887-47ed-addc-3be664dd7053@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: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Queue-Id: E56321F79D X-Rspamd-Action: no action X-Spam-Flag: NO 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)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCVD_TLS_ALL(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; 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)[imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,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-Score: -4.51 X-Spam-Level: On Tue, Jun 17, 2025 at 02:10:09PM +0200, Oscar Salvador wrote: > On Tue, Jun 17, 2025 at 02:08:16PM +0200, David Hildenbrand wrote: > > folio lock is a sleeping lock, PTL is a spinlock. :) > > Lol yes, overlooked that totally. > And I also saw the comment from mm/rmap.c about lockin order. So, we could do something like this: if (folio_test_anon(old_folio)) { spin_unlock(vmf->ptl); folio_lock(old_folio); spin_lock(vmf->ptl); vmf->pte = hugetlb_walk(vma, vmf->address, huge_page_size(h)); if (unlikely(!vmf->pte || !pte_same(huge_ptep_get(mm, vmf->address, vmf->pte), pte))) { spin_unlock(vmf->ptl); folio_unlock(old_folio); goto out_take_lock; } if (folio_mapcount(old_folio == 1)) { if (!PageAnonExclusive(&old_folio->page)) { folio_move_anon_rmap(old_folio, vma); SetPageAnonExclusive(&old_folio->page); } if (likely(!unshare)) set_huge_ptep_maybe_writable(vma, vmf->address, vmf->pte); spin_unlock(vmf->ptl); folio_unlock(old_folio); goto out_take_lock; } VM_BUG_ON_PAGE(folio_test_anon(old_folio) && PageAnonExclusive(&old_folio->page), &old_folio->page); spin_unlock(vmf->ptl); folio_unlock(old_folio); spin_lock(vmf->ptl); vmf->pte = hugetlb_walk(vma, vmf->address, huge_page_size(h)); if (unlikely(!vmf->pte || !pte_same(huge_ptep_get(mm, vmf->address, vmf->pte), pte))) return 0; } Hopefully we can do some refactor here, because I quite dislike the unlock-lock-retake-unlock-blah cycle. -- Oscar Salvador SUSE Labs