From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wm1-f48.google.com (mail-wm1-f48.google.com [209.85.128.48]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6114A481FDA for ; Tue, 12 May 2026 08:02:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.48 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778572957; cv=none; b=mr9pgAJhPhsxeipQSA8zKAcTH8PpBf8u57It60mmG6roaWuH4YMhjaNcClPhG8D2aXMriPZoco0iabbYFGRsPwrJkoxpv4NOkg1PJEwXUYrdQzKn/rNraDF7FiNRj/I3jW7eXdT3iue7M3MMHxVXfnU8b8jGrVL6yP57uWef2PM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778572957; c=relaxed/simple; bh=VR+q0DFaiAqBsEjuXbpXoAptjSvweVDv+3Uou/jgT+w=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tVdKdprzL2IoVitynQetlILqLnMykdbWGR+0ZApBZZcBDWu1SMiUiGE5xHbN/82NAMMbxo4tDZZnFT2uy/6OI4oqCaz5KgA1ZBhs/RT2yG2OkUAYQQn1wHb2LvBj9eKgp+XVCyWG28k8EoGKi9wf4x1d2QpC0mlw1Jt8HzuMVQw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=U7H5OZIj; arc=none smtp.client-ip=209.85.128.48 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="U7H5OZIj" Received: by mail-wm1-f48.google.com with SMTP id 5b1f17b1804b1-48374014a77so48128155e9.3 for ; Tue, 12 May 2026 01:02:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1778572954; x=1779177754; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=NfRcnEmG4L1C60ic4jCNkQQaD5s9vtpkTF7+A4mU6Uw=; b=U7H5OZIjztGIuXhVYqbOP+y7sB9gJCqBJzj/eqsNAar856Q+dF287lsB/gx30xHlEm f0CaPdr5DcA41DSca+bYcx7KLyrUw/evP5/BGQf2e/OG4S81Tdgg5lN2kfO/tsVnP8+e X60bWKejW8ms3S69566b8K+WPzB3A+LfKjlH7CDXJpCkRquQ3nwzch7GHCPJFA9zuNbm /zHtfla0/QPbOIVZP5hgwfDznIuxqtvGUilYFq0cHonfgrujlTkspcckB9HBrzi7ZYWc uMQfI8jBUjyc21T84dYY0jeE/HfjK0z6JszlEGgojeMiWT3zqRBtEoR0VsAfkg3LMcI1 pFjA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1778572954; x=1779177754; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=NfRcnEmG4L1C60ic4jCNkQQaD5s9vtpkTF7+A4mU6Uw=; b=MwMbqWL2DI1psaSVArS4J/CSz7n8YomKku6xEAVsTW+0tAKCxmZC3bIK/FcEZ3cC4e AR9S48Qklvv0g60qDe1VYPkb7+2GxVEzZu0OyzTMv2snIAKVKvd8oNMOnsMqk0XPqbqL BWj42mHq3olRp3jof2e/kZnw/5RHFWmfVO+4B9PeDMj6OnRlu3PYB3frsF1M+VswQBxq ecTu+Bx+rWV5gglTJ8FKOiEREPxbcHHlOHv1xiz370vNBSJtTCM3ViX/yCzVPuRLlZ7A rQ+HFC7Bg2jzoj0luG6zBsjpJXq0vMHUpUfhEb4K1ksKkqOE7aGpYuJHPED1ehHX9/x/ /wRA== X-Forwarded-Encrypted: i=1; AFNElJ8bxSYDbnnwcmfwUa4MkVv42fhV/V18xBvxi3Ef/FLTxEqoY0A6oYvYI/l1taRR7tRVi/dVNWWgi4Cj4Hc=@vger.kernel.org X-Gm-Message-State: AOJu0YyiadQfnzCVOyVxZ6ztuKrXU2fQyBqrnDMD7gpZcIeyKDR4osqV 9Ecc930xUYZgnCUTNmHFCm57dc+hyHdIEurm03j35ZP27DH2wAq8NIRy X-Gm-Gg: Acq92OHinhuzs32x28u5Fa4zjsnHfK9x2Dm3rYbScXsz2oG8NwJ4585XEyfdhxXhO08 rlaNBvWUM/AOlAOQpZsqR9ohr4NDxeL+x4ssXVDP9LosM4W3pX9EnQ+zwev4Y2hy1ujAP6VGT2z oIZaEKXSeJQfhdMm3N1mm4pbizsDS97YDfXDxgZ2NrveCQdJd7Y1KXgFbML4/paCEiZpYPata7p lyXn2NWE3JSO0W6FcL7kaSHNVpQsVBgs8mo0ZTCaUDJlq/VG7B1qzzrqXNBgasxQ9YZE3NvVNXC MNdgsKXCeMFIF6zDDp+O5mKR/0ZC5yg84pWaaATqJuznMrBQHutEGVq8xcvx4ONPcpabTyu2XPD yGeTHuczDJeOCJs5ssAgtZWPk8JZwyld5zlS2f6+TdhNYyRdce8QzL3+LXblpJioTSLYyHPYqhZ tz5GnJXaSH0XFRIYQ5UHCptdnqreaTz8wtRj4UNntgRoClLe5rZr15cLwgkqhXfOybLCt8OdeH9 XYiN1Xi3rV1G6mAAi7sPlgkhnf1eI//8u/TtA9Xt8zX X-Received: by 2002:a05:600c:4695:b0:48e:5d91:cffb with SMTP id 5b1f17b1804b1-48e706c0662mr199733775e9.10.1778572953433; Tue, 12 May 2026 01:02:33 -0700 (PDT) Received: from fedora (cpc92878-cmbg18-2-0-cust539.5-4.cable.virginm.net. [86.16.54.28]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48e906a06d1sm30409515e9.1.2026.05.12.01.02.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 12 May 2026 01:02:32 -0700 (PDT) Date: Tue, 12 May 2026 09:02:30 +0100 From: Vishal Moola To: Tal Zussman Cc: Jan Kara , "Matthew Wilcox (Oracle)" , Andrew Morton , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/filemap: fix page_cache_prev_miss() when no hole is found Message-ID: References: <20260510-prev_miss_fix-v1-1-755bb123145a@columbia.edu> <5280b6f9-e6d7-4854-bbdc-ed5349a478ed@columbia.edu> 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: <5280b6f9-e6d7-4854-bbdc-ed5349a478ed@columbia.edu> On Mon, May 11, 2026 at 02:15:41PM -0400, Tal Zussman wrote: > 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. I prefer Jan's suggestion, and making page_cache_next_miss() consistent with it sounds good :). > 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; > >> > } > >