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b=PT7fNisghbGkV2E0eOjvhLTYToAx/YhsPNvuhceXEyQempy0KPL+oBWgiyzyG3TCL fjEEfBzUuUE4t8kD1wlr0ja8C01RcYkezsMXpzCrXn7AyUbgC2JNVN79uMn5io/FlP W8cdM0g6U+uAemBKJPbCn9459uuyR2O0s3M88pdax/OdSD0OLxGgbcq1uUwImG7jB9 OEoZaRCkjmeVWAbxRSYIdZfMeduljQimmPqVr+cFwbkYy76wTojEqbtH9tJsYkgGdT khgTQEFm6RBO30SVMi21RgXD/yPmskpHUrM/ClP0eTC4mnEuogpZbb/D8nHgio9hRS lhmBT5uPgIxqA== Message-ID: Date: Wed, 29 Apr 2026 16:40:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/khugepaged: return -EAGAIN for SCAN_PAGE_HAS_PRIVATE in MADV_COLLAPSE To: Dev Jain , Vineet Agarwal , akpm@linux-foundation.org, ljs@kernel.org Cc: 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, baohua@kernel.org, lance.yang@linux.dev References: <20260429140434.439456-1-agarwal.vineet2006@gmail.com> <70b44146-4549-479f-8d31-b40a8e747c60@arm.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 97DFC80014 X-Rspam-User: X-Stat-Signature: cqnipdjwwjzhiyjpj3y9383posbs4sif X-HE-Tag: 1777473637-689712 X-HE-Meta: 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 Vv1iZPBn AWSu3qhVB1YtLY5UehNbV0wD83Nhp6cZnF+BfNJh3ne+e/saKKSKxv50llqHuL6ERjaQ6nRRKk6cGOh/DioMThLBItNtO4T8m4moS2hn4G2GaF79obDiXAGHMYJ8BgJ0c6s5Z/Bw62vcFQbQtQT5yxwK7hJHah00WOSWtp9jAsuBCZ7hQpaQahcIadsBzcYsiUjFrjzk3ONxAyTFc3+2sh8Vcs3BwSBhj46QrG0qhC1zSdE3hJrzwUbOJehJJdy1fwAz1fy8+z/Kj4/dsphrKXroT8UYcQuFnxLH+LAJWWFtUcZWSIp2N6yrHnMIUl79+KzlF7inK02DmecoWCyyueFbmVhVE6nvI3Xwn6AYiA1sP3F/9Y+O0gTpjohojq7GFHCcuamRHg/PH/vj2FSFlBaoHFp7gyv4+gc+AOdP5QqCndGDmjtP4/05jWc16bRLZPy9mGja1eyESyMWIrhLGHBPZxPtWwCcmqZj2MNM3IUdLRt4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/29/26 16:20, Dev Jain wrote: > > > On 29/04/26 7:34 pm, 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. >> >> Signed-off-by: Vineet Agarwal >> --- >> mm/khugepaged.c | 1 + >> 1 file changed, 1 insertion(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index b8452dbdb043..58f55115d7d8 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -2808,6 +2808,7 @@ static int madvise_collapse_errno(enum scan_result r) >> case SCAN_PAGE_LRU: >> case SCAN_DEL_PAGE_LRU: >> case SCAN_PAGE_FILLED: >> + case SCAN_PAGE_HAS_PRIVATE: >> case SCAN_PAGE_DIRTY_OR_WRITEBACK: >> return -EAGAIN; >> /* > > https://lore.kernel.org/all/4ab54ae0-2607-443d-8698-788d8e951bdd@arm.com/ > > I had mentioned at that time, other error codes may fit the -EAGAIN > (or something else) definition as well, for example, probably SCAN_TRUNCATED. In folio_split(), we such events as a mixture of EAGAIN and EBUSY. filemap_release_folio() errors are, for example, handles as EBUSY. So in that regard, using EAGAIN here (as we don't use EBUSY) makes sense for SCAN_PAGE_HAS_PRIVATE. -- Cheers, David