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Mon, 11 May 2026 11:15:42 -0700 (PDT) X-Received: by 2002:a05:690c:6983:b0:7ba:a726:57a4 with SMTP id 00721157ae682-7c10567accdmr99156077b3.36.1778523342322; Mon, 11 May 2026 11:15:42 -0700 (PDT) Received: from [10.206.149.208] ([129.236.224.3]) by smtp.gmail.com with ESMTPSA id 00721157ae682-7bd66019105sm153053607b3.0.2026.05.11.11.15.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 11 May 2026 11:15:41 -0700 (PDT) Message-ID: <5280b6f9-e6d7-4854-bbdc-ed5349a478ed@columbia.edu> Date: Mon, 11 May 2026 14:15:41 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/filemap: fix page_cache_prev_miss() when no hole is found To: Jan Kara , Vishal Moola Cc: "Matthew Wilcox (Oracle)" , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260510-prev_miss_fix-v1-1-755bb123145a@columbia.edu> Content-Language: en-US From: Tal Zussman In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Proofpoint-GUID: Driy1S4WX7xLfbrM-FR7UPSC-FffYAL4 X-Proofpoint-ORIG-GUID: Driy1S4WX7xLfbrM-FR7UPSC-FffYAL4 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTExMDE5NCBTYWx0ZWRfX0orovyW1oT4t j28r04PljpxIXzu0B0W3dR8auA2U5N63MzGjacEn+Lh1xFzHWHGDRnip9tCogj+bzU/OVbElLbB uS+s626FWxkm8BLpkJ6WFPm1gL8/1ykTzWMowXkUurGsK+K4D4RyM7nkP5ua97PQ6XGwWRSf+P+ S6J4/x/0uuGP0Z0afEOJzrkevcVqtoj+XHekAE6SOhfpncmylaYfGO1XpDTX0pCHnpH6G8z4z+F BMSnRaYFRq0q1AOF7UpKW4erOGDw9Onn7f64jIzlPEDUA82y/vLzKv2J1B+XzfC/0dsrLGVr95l n+miZsUvuAoNOzf3bPGwbst1UdQUzHgM80b7jYpovZ/HIPBYkpPKYBr1CITt87BQnUSkQ57Zd+Y XvaHKI6hSCiVjA+zKNlBl/lOvhppUzoK42oXmERJLOS3+k3fWmCpe8A3rVNyygzR6NpYhrIU0RW 90Nj270Mg4343X6Vs/A== X-Authority-Analysis: v=2.4 cv=Eeb4hvmC c=1 sm=1 tr=0 ts=6a021ccf cx=c_pps a=g1v0Z557R90hA0UpD/5Yag==:117 a=uvjyHnA+wHnVBzamt/20/A==:17 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=x7bEGLp0ZPQA:10 a=VkNPw1HP01LnGYTKEx00:22 a=Da8U98TiO7q1upZEImrf:22 a=svvvyxlR1OQQkelhaPoB:22 a=5NEz3i_17GglibqS5w4A:9 a=QEXdDO2ut3YA:10 a=MFSWADHSvvjO3QEy5MdX:22 X-Proofpoint-Virus-Version: vendor=nai engine=6800 definitions=11783 signatures=596817 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 impostorscore=10 malwarescore=0 priorityscore=1501 clxscore=1015 lowpriorityscore=10 suspectscore=0 bulkscore=10 adultscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604200000 definitions=main-2605110194 X-Stat-Signature: 9ekhxgtqq5ukpygtkewt3fz15xfs5kew X-Rspam-User: X-Rspamd-Queue-Id: 64C82C0013 X-Rspamd-Server: rspam07 X-HE-Tag: 1778523345-14880 X-HE-Meta: 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 QiHeqHNM +iDkoTJnOIM7Cy76qxFDc4E5ZA6YR0M5uSczlUVHWxnDLGHq/QtlzaDUyM6UYmYLxjl+IZjf3tgXuwMrO42SjrL7A+R7xYzDllviYIi05wHddswXuF1TaQM0oQb6RLR3CopGy9EAWZ9WueDjSsienzR5tF7FfinPdr5Ug/RNqiOv/gpWei1q1xaANGxvSyhYKS3751I6JRmbgbDdgxmLccpNx8dUt/J08gUB+pCacQPzaDVpxJJCCh1b8xcxkCSGlZtd93wk4xG7NCRY46SQz57xkZtJjgm7b4jt4yldShhXyRY5m7psyG+6/8NG4wrlUxc6gwv1ArVMYAA6cGaKSH1OniPMyp6m2fge4XixoRdkFeVkRxlblnsPcC3HQy355vlpjDm11K4p5Kz52q+RiJUaXkkxBYoSBkV/+cjoFTTnJgVFkQtBKcJaembhleroW5pMgfTgswKBaN2UojvvX89JqWWpJGMvXR5nWcHBUKV2YVUsv6XCRc55qxd5GdFvyNhgjRERxVLeIAG/Eoxuga5CLZM2LRBbrFRRli4uOxLO0sdNY3lm7N/GELMW7Fyy07Xh1Q4P8cV4dKLpNssmGhiOG9Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/26 12:26 PM, Jan Kara wrote: > On Mon 11-05-26 13:44:17, Vishal Moola wrote: >> On Sun, May 10, 2026 at 05:54:17PM -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. >> > >> > Mirror the fix in commit bbcaee20e03e ("readahead: fix return value of >> > page_cache_next_miss() when no hole is found"): preserve max_scan in a >> > separate variable across the loop and return `index - max_scan` from the >> > no-gap-found path. >> >> IMO, this way of fixing it hurts the readability. I'd prefer something >> similar to the fix in the original commit. Or... >> >> > - while (max_scan--) { >> > + while (nr--) { >> > void *entry = xas_prev(&xas); >> > if (!entry || xa_is_value(entry)) >> > - break; >> > + return xas.xa_index; >> > if (xas.xa_index == ULONG_MAX) >> > - break; >> > + return ULONG_MAX; >> > } >> >> If I understand this correctly, couldn't we just do something like: >> if (!max_scan) >> return xas.xa_index - 1; > > I think the easiest to understand would be to do the above two explicit > returns instead of 'break' and change below to: > > /* Return start of the range - 1 when no hole is found */ > return xas.xa_index - 1; I can do that, but I think it should be consistent with page_cache_next_miss(), which does index + max_scan. If the xas.xa_index approach is preferred, I'll change it in both functions, and also get rid of nr. The nice part of 'index - max_scan' is that the kdoc describes the range in terms of that already. Thoughts? >> > - return xas.xa_index; >> > + return index - max_scan; >> > }