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[86.16.54.28]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48fc8d19974sm52175545e9.2.2026.05.13.00.53.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 May 2026 00:53:00 -0700 (PDT) Date: Wed, 13 May 2026 08:52:58 +0100 From: Vishal Moola To: Tal Zussman Cc: "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/filemap: fix page_cache_prev_miss() when no hole is found Message-ID: References: <20260512-prev_miss_fix-v2-1-4af8e5c1ae62@columbia.edu> Precedence: bulk X-Mailing-List: linux-fsdevel@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: <20260512-prev_miss_fix-v2-1-4af8e5c1ae62@columbia.edu> On Tue, May 12, 2026 at 04:45:59PM -0400, Tal Zussman wrote: > page_cache_prev_miss() is documented to return a value outside the > searched range when no gap is found. However, the no-gap-found path > returns xas.xa_index, which after a successful loop is the first index > in the range. As such, that index is misreported as a gap. > > The sole caller, page_cache_sync_ra(), uses the return value to estimate > the cached run preceding a sequential read. In some cases, the buggy > return value can undercount the contiguous range by one, shrinking the > readahead window or pushing borderline requests into the > small-random-read branch. > > Fix this by returning the start of the range - 1 when no hole is found. > Update page_cache_next_miss() for clarity as well. > > Both helpers were previously fixed together in commit 9425c591e06a > ("page cache: fix page_cache_next/prev_miss off by one"), but the fix > was reverted because it caused a hugetlb performance regression. hugetlb > no longer uses these functions and next_miss was subsequently refixed > in commit 901a269ff3d5 ("filemap: fix page_cache_next_miss() when no > hole found") and commit bbcaee20e03e ("readahead: fix return value of > page_cache_next_miss() when no hole is found"), but prev_miss was not > addressed. > > This was found by pointing Claude Opus 4.7 at mm/filemap.c. > > Fixes: 0d3f92966629 ("page cache: Convert hole search to XArray") > Assisted-by: Claude:claude-opus-4-7 > Signed-off-by: Tal Zussman > --- Reviewed-by: Vishal Moola