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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770633720; bh=ywJCRbCgffCqqfChHalI2gIpjQYUmGoILfzVpNH7JE0=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=S5DYD3/rPRNhXUdylNPOOMeREFxP8EedkjL2Dg/gOs00I8eQtLP0qPh/M4UxzV9ab CArMqgdfjrcXuo+UI5gA7uiE0brOcn+HSbyjc84vW7Pnq7LaHpPgpBqOutHaa3XWZr W/5JXn4Ts0RRjiQbnl6rFqMct6lrc/w2ezbWVbpYS5hVs62XsnNR9ap62QdkBH5bHz qVezcj4G9FLHPoFT9vdzw+d2AvbYpafurb4QCZfGIss493gctvno/ipg9yDTMlHiZ5 BTkbVXw5ZRnC8d8AC7YacM5Vsk4Dj6py8U942CFVad9c+3bQjFFA91UyKbUcxGm1lJ X0MSJ5w3+cOSA== Message-ID: Date: Mon, 9 Feb 2026 11:41:52 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled() From: "David Hildenbrand (Arm)" To: Deepanshu Kartikey , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, ackerleytng@google.com, seanjc@google.com, pbonzini@redhat.com, michael.roth@amd.com, vannapurve@google.com Cc: ziy@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com References: <20260209033558.22943-1-kartikey406@gmail.com> <0d9cada8-7148-4a5c-a09d-120ef54559d7@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 36DE68000D X-Stat-Signature: 7b3u1waj4yn4hk14fmbf9y3ti5n8kuxr X-HE-Tag: 1770633721-148127 X-HE-Meta: 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 DtXx311A nISCaQYcTMcrN2ZGUJvORwlFrI836hoL/BCgawTYQc0zj/2MrRwf1ffWCw9lUfQO10zTfmyfEjeUlzyw3HdH0rR+5M6p+5KPLlQ1OGdRtC5VW9Vs= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/9/26 11:24, David Hildenbrand (Arm) wrote: > On 2/9/26 04:35, Deepanshu Kartikey wrote: >> file_thp_enabled() incorrectly returns true for guest_memfd and secretmem >> inodes because they appear as regular read-only files when >> CONFIG_READ_ONLY_THP_FOR_FS is enabled. This allows khugepaged and >> MADV_COLLAPSE to create large folios in the page cache, but their fault >> handlers do not support large folios. >> >> Add explicit checks for GUEST_MEMFD_MAGIC and SECRETMEM_MAGIC to reject >> these filesystems early in file_thp_enabled(). >> >> Reported-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com >> Closes: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44 >> Tested-by: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com >> Signed-off-by: Deepanshu Kartikey > > So we were able to reproduce this with secretmem, right? > > We want to add "Fixes:" for the introducing commits, which would be he > commits that enable secretmem and mapping of guest_memfd pages to user > space. Can you identify them? > > And also > >     Cc: stable@vger.kernel.org > >> --- >>   mm/huge_memory.c | 3 +++ >>   1 file changed, 3 insertions(+) >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index 40cf59301c21..4f57c78b57dd 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -93,6 +93,9 @@ static inline bool file_thp_enabled(struct >> vm_area_struct *vma) >>           return false; >>       inode = file_inode(vma->vm_file); >> +    if (inode->i_sb->s_magic == GUEST_MEMFD_MAGIC || >> +        inode->i_sb->s_magic == SECRETMEM_MAGIC) >> +        return false; > > That's nasty. We want some way to identify that through the mapping. > > Unfortunately CONFIG_READ_ONLY_THP_FOR_FS ignores any > mapping_set_large_folios() configs by design. > > And CONFIG_READ_ONLY_THP_FOR_FS might go away soon, but we need a fix > until then. > > While we can identify secretmem through vma_is_secretmem(), we can't do > the same for guest_memfd as it's built as a module. > > Unfortunately AS_NO_DIRECT_MAP[1] won't work. > > Maybe introduce a AS_NO_READ_ONLY_THP_FOR_FS, which we can just easily > rip out along with CONFIG_READ_ONLY_THP_FOR_FS later? On second thought, why do we pass the !inode_is_open_for_write(inode) in file_thp_enabled()? Isn't that the main problem for these memfd things? Maybe a get_write_access() is missing somewhere? -- Cheers, David