From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 69A9D175A64 for ; Wed, 29 Apr 2026 14:40:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777473636; cv=none; b=oF7I/pqMnEMehYvToqDgi+iJCcQ6HDp1XwZDiQ/Ix0dJmRzWDZiXeW+i4kxTHvXJnPWWRV9omMJ9hhII1WPJ/A3aK44VQZr30/XHFXPPvftIMMMGJl7uCWtckfSzSFN4EBQJNxJqVsGLLhAiecHIUaAalrUgJt70hUaJaTLLLEQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777473636; c=relaxed/simple; bh=TpGwJcYj+6XBZyxmMOS7BWdtqilX7jFLEjuPIRCbrWU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=J7NMektTfPHY6E2ZoQK31Mdhi3QkOj+3YiYvVLkNjvHhVWhmpqxxkSn8Bi92gO7Drxf1G5bVX29406D6BC8uc2TJtnbysdGK6gJZip4YKWZnhpWw9ISs1wwmANPHnmLjRMyU4vy/IJ58eIibFPCRAfTGlUNzqwTaRaZ5XyQoySw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PT7fNisg; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PT7fNisg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D200EC19425; Wed, 29 Apr 2026 14:40:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777473636; bh=TpGwJcYj+6XBZyxmMOS7BWdtqilX7jFLEjuPIRCbrWU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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