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b=ZT7h2jKtezIYqQe+ESZxeDe1Z3QaoB2gaXbMeIPypYqY+hjd31tIw/88Ipnzt94/M vSWqGo4F+8jJCNVmxIe8O4Ydv5j1TPp1ez6ix3/6ErT4+WIMDGEcXYWZcy0x0dVTcF SLChFWxLI7k996tm7v3moitsx9QGpD7DRfSJPw7A= Date: Wed, 29 Apr 2026 07:57:24 -0700 From: Andrew Morton To: Vineet Agarwal Cc: david@kernel.org, ljs@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev Subject: Re: [PATCH] mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE Message-Id: <20260429075724.a97a964003f624e47d9c0a54@linux-foundation.org> In-Reply-To: <20260429140434.439456-1-agarwal.vineet2006@gmail.com> References: <20260429140434.439456-1-agarwal.vineet2006@gmail.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: hriu6rdof4ekwmwssyjo98uc51w3riiz X-Rspamd-Queue-Id: 1E30DA0004 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1777474645-803021 X-HE-Meta: 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 iSUbaURN YgrcbNzVNRUB3+ygBXAMZWcjYVqZCVTPoiFMi3lhpjhYS+rGbFUCfCkHh/hEzJJn7lhYtnD/Cgx2JUb65MmmFvBt3NypnFiLHSO/i6kbRoi9oO+n3+iqWreCx0bc4Fs28EASEkHQYEqfFEZ6KF/doj5dSRNtg9IXgKb5DGQRFHDyl+urdXDJM6dgX+CoEs3Om7l/gJDKDEDbklEsBf6OVz774lJvy6V5LnzQwXI7c/s/WHxNh09fxHY95lfHzT/SNDnGore4os3dQ02JHgw4LlbcZ7tNllOvfhlaVyZ86OOlopXSP+T67ZRbPzFtH2jp7s2l3a9WkjNtcrFvMolkRhfBbpng1DolBYLLJHSK8Aco5TzKCgLVf07iHkha8vcRh7JyhitregflWB7U= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 29 Apr 2026 19:34:34 +0530 Vineet Agarwal wrote: > MADV_COLLAPSE uses errno values to provide actionable feedback to > userspace. Temporary resource constraints are mapped to -EAGAIN so the > caller may retry, while intrinsic failures of the specified range are > mapped to -EINVAL. > > collapse_file() returns SCAN_PAGE_HAS_PRIVATE when > filemap_release_folio() fails while isolating file-backed folios for > collapse. This currently falls through the default case in > madvise_collapse_errno() and is reported to userspace as -EINVAL. > > However, filemap_release_folio() failure commonly reflects temporary > folio state rather than a permanently uncollapsible range. > > For example, ext4 returns false when a folio still has dirty > journalled data, btrfs returns false for dirty or writeback folios > before extent state release, and NFS may return false while reclaiming > filesystem-private folio state. > > In such cases, retrying MADV_COLLAPSE after writeback, reclaim or > journal progress may succeed. This matches the existing -EAGAIN > handling for SCAN_PAGE_DIRTY_OR_WRITEBACK and other transient collapse > failures more closely than -EINVAL. > > Therefore, map SCAN_PAGE_HAS_PRIVATE to -EAGAIN so userspace receives > retryable feedback for this temporary failure path. Seems very reasonable to me. The madvise(2) manpage could be a lot more helpful here. EAGAIN A kernel resource was temporarily unavailable. and that's it!