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Wed, 22 Oct 2025 07:08:44 +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 M23uL/yC+GhCAQAAD6G6ig (envelope-from ); Wed, 22 Oct 2025 07:08:44 +0000 Date: Wed, 22 Oct 2025 08:08:42 +0100 From: Pedro Falcato To: Kiryl Shutsemau Cc: Andrew Morton , David Hildenbrand , Matthew Wilcox , Linus Torvalds , Alexander Viro , Christian Brauner , Jan Kara , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Kiryl Shutsemau Subject: Re: [PATCH] mm/filemap: Implement fast short reads Message-ID: References: <20251017141536.577466-1-kirill@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251017141536.577466-1-kirill@shutemov.name> X-Rspam-User: X-Rspamd-Queue-Id: 802E220003 X-Rspamd-Server: rspam03 X-Stat-Signature: 1cizftjbixsem4yy94o3ijpq8dftg3rk X-HE-Tag: 1761116935-543618 X-HE-Meta: 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 I7Y9+KfE 7nlKVOu8WahHZOvlQc52Djb0mtl34VA/3wsx5mi1btIh81Icf8/D3+hMQe4czK4DtYoxBl9b8QmKzWvYQBktqnJANbANKZZeQrI3KN6vQ+ArCeRzmNhrQJtzzk0SGBr69drCz5O4+ZV5QFq78W5U6bfcwoVwS+dJY5a8rygv1YWb5Zq4+dSAFWGLJKZT6itHpBxSSi/DfbDQwVPZ+/ml51SiaKTXZaxOQCyMFLR2tLyAO9/qCcpQm4sOutAHA/8dehw8X1eUOeWMC5NqRxBHZDuQgbw1eBDDvvl7g5AzhUz/mg+iw/q2tMyUt3Nh3Nrk+kG+ZB2MKI5hbkKg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Oct 17, 2025 at 03:15:36PM +0100, Kiryl Shutsemau wrote: > From: Kiryl Shutsemau > > The protocol for page cache lookup is as follows: > > 1. Locate a folio in XArray. > 2. Obtain a reference on the folio using folio_try_get(). > 3. If successful, verify that the folio still belongs to > the mapping and has not been truncated or reclaimed. > 4. Perform operations on the folio, such as copying data > to userspace. > 5. Release the reference. > > For short reads, the overhead of atomic operations on reference > manipulation can be significant, particularly when multiple tasks access > the same folio, leading to cache line bouncing. > > >+static inline unsigned long filemap_read_fast_rcu(struct address_space *mapping, > + loff_t pos, char *buffer, > + size_t size) > +{ > + XA_STATE(xas, &mapping->i_pages, pos >> PAGE_SHIFT); > + struct folio *folio; > + loff_t file_size; > + unsigned int seq; > + > + lockdep_assert_in_rcu_read_lock(); > + > + /* Give up and go to slow path if raced with page_cache_delete() */ > + if (!raw_seqcount_try_begin(&mapping->i_pages_delete_seqcnt, seq)) > + return false; > + > + folio = xas_load(&xas); > + if (xas_retry(&xas, folio)) > + return 0; > + > + if (!folio || xa_is_value(folio)) > + return 0; > + > + if (!folio_test_uptodate(folio)) > + return 0; > + > + /* No fast-case if readahead is supposed to started */ > + if (folio_test_readahead(folio)) > + return 0; > + /* .. or mark it accessed */ > + if (!folio_test_referenced(folio)) > + return 0; > + > + /* i_size check must be after folio_test_uptodate() */ > + file_size = i_size_read(mapping->host); > + if (unlikely(pos >= file_size)) > + return 0; > + if (size > file_size - pos) > + size = file_size - pos; > + > + /* Do the data copy */ > + size = memcpy_from_file_folio(buffer, folio, pos, size); > + if (!size) > + return 0; > + I think we may still have a problematic (rare, possibly theoretical) race here where: T0 T1 T3 filemap_read_fast_rcu() | | folio = xas_load(&xas); | | /* ... */ | /* truncate or reclaim frees folio, bumps delete | | seq */ | folio_alloc() from e.g secretmem | | set_direct_map_invalid_noflush(!!) memcpy_from_file_folio() | | We may have to use copy_from_kernel_nofault() here? Or is something else stopping this from happening? -- Pedro