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b=qqpBBoRAPpFtdbygI5te+6DMFR2ULPHp+mdHIgT7EXp5Ap0QmtmsaVE+8wVktWG7+ E3EpkARf0310WL7WpvBXnUKYlGaOD50KEK3ESWBcHpHb5blEZu+M32HYLhlg8DcoWL /OJiRimM3kzA87J3x5V6QH1Yx0nWn2el52RYxAOxFegRifD41A/thNtnLEQl+T6xwv uqr4uW0cugJ3igO3b/PuLaScCEl3co3p9oZp/bvOb+vMG03MkXPvwHWdmw8PJeY+t1 SHgcb40tthApNsYn4G+c4qT4Wco7grALBDJWgNgb8CbwNIDG99mIBBAYgkLL+Y+MaT ibJnSp0MVoqPQ== Message-ID: <238ef4ab-7ea3-442a-a344-a683dd64f818@kernel.org> Date: Tue, 6 Jan 2026 15:33:54 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/1] fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes() To: Miklos Szeredi Cc: Jan Kara , Joanne Koong , akpm@linux-foundation.org, linux-mm@kvack.org, athul.krishna.kr@protonmail.com, j.neuschaefer@gmx.net, carnil@debian.org, linux-fsdevel@vger.kernel.org, stable@vger.kernel.org References: <20251215030043.1431306-1-joannelkoong@gmail.com> <20251215030043.1431306-2-joannelkoong@gmail.com> <616c2e51-ff69-4ef9-9637-41f3ff8691dd@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: jz95rk1cky6eb95oertgqfrxpkfgu4d7 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 5C8B718000E X-Rspam-User: X-HE-Tag: 1767710041-563344 X-HE-Meta: U2FsdGVkX19K/Z+00VLQyZSigvWMlM5r0JtWt6v1xf/JgxawhNylp4GVeeeFF0UgsoQAPVLCpDFvknCgNQDqXE7t/Abh8/rKJ2HpPJNIIPjEetFqSU/A1N1TRtfWGQeXbcM24HS+nIttp5ecnSEqOErnTY6T9as4eOMekvbfJMWseVMjIZn5fzvA9RpMTicGVod77ay1c6X1UyHkpzjdlmK9Aj7f2BIbujxEaZqycQPbcJEXsegMXbRwN3R8OJAAE5a5NgKYqeVQQjN9SifcdHwytuBI5/b7BmPOBW4Jq6Nrq4JiTmxXzP9poerFNT66R1jBTEkqRbdY0k+/5bNBRfH4F5jyR1pAr6V5ebkrrEySXkjQ5nA7h6dhTVThLUPCRgybt2+TkqMxY4bvgfjbCicd3soc+QmN1mNRfwiTy97tS9VmOnyEo6T1LnzLS+ethfvlPFyKxekhb+e7ycQiif4jKuTKf8hPsvRiuuvxMvyEI4qHkroLFLtjc1jy8Qkt37ORz9tQmUxjBO8IsN6E2ujuledXXXalGI5h37C6m6G2YwX/pEicgqeNszYEN1Q2KFfVOFojqrmR8Zhg9PZNKDWFSNVacIbyQeXzo2sLDhF8aiVRPVLrIp3HWPPMJGC6A2UvegTdMhJxU1e4UEOt9CPDXnPI1MScjjbwDi/plLoO/u5D1lGTs62QdAPOLTej3Mi1uKHzhwiO3rGmWkUTyUuAqJug5Q/8gFa94Z/lzvDBqhZe9gMhtGPdSNMplHMeoKiv2URohZQJ6oicTNx7EZ9MRynI3sgMxCzx/Ug2xORGq6OPZnngWqIJRgxtT5/AFhdN50YBMla4p/jU/dEhGvvzDln81QRMswY9+mZBAI1bFRtRRQJuNjyQ4k6EHephsaSKPHpoBdUrPg6dCWRAZ9I+eQq3GmT9TtfCOXu/l8vyCy8yNePTC/nBkRYehSdccbDGoy/cqr36Axjb9Fk Tm7ypTwr //tlYPnNIeLLqtfESy6t1tOzHnIlATotSSwMEjY1I3fCxs8MFSNRsR+mkWsKHrbEltGgKdqysvdxWQuYJKV9SyAftTRWcw4fTzmpVpNVk3knXwDTRXiDEpFzMMsIiU5Ro5qqBJmM89PFh8gZfBSFNlVy79wbBKDKGSSNLTGQLeW59VikjwBhYTu0auBit26HgVGN0k+KyyAgEMThafPwrFrWhwHa+wFHgGKnstnQ1p4Ak1kV39JNJYfh1+9C/3AFq33yT1D5Nix8LJgmH+8uPIiwn8I3VAg8aliGDSYUWx9/W0KnLnM40WRk82B8kiyG9Y4KJ4i4GYXPb6zk= 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 1/6/26 14:13, Miklos Szeredi wrote: > On Tue, 6 Jan 2026 at 11:05, David Hildenbrand (Red Hat) > wrote: > >>> So I understand your patch fixes the regression with suspend blocking but I >>> don't have a high confidence we are not just starting a whack-a-mole game > > Joanne did a thorough analysis, so I still have hope. Missing a case > in such a complex thing is not unexpected. > >> Yes, I think so, and I think it is [1] not even only limited to >> writeback [2]. > > You are referring to DoS against compaction? In previous discussions it was raised that readahead runs into similar problems. I don't recall all the details, but I think that we might end up holding the folio lock forever while the fuse user space daemon is supposed to fill the page with data; anybody trying to lock the folio would similarly deadlock. Maybe only compaction/migration is affected by that, hard to tell. > > It is a much more benign issue, since compaction will just skip locked > pages, AFAIU (wasn't always so: > https://lore.kernel.org/all/1288817005.4235.11393.camel@nimitz/). > > Not saying it shouldn't be fixed, but it should be a separate discussion. Right. But as I pointed out in [4], there are other call paths where we might end up waiting for writeback unless I am missing something. So it has whack-a-mole smell to it. > >> To handle the bigger picture (I raised another problematic instance in >> [4]): I don't know how to handle that without properly fixing fuse. Fuse >> folks should really invest some time to solve this problem for good. > > Fixing it generically in fuse would necessarily involve bringing back > some sort of temp buffer. The performance penalty could be minimized, > but complexity is what really hurts. I'm not sure about temp buffers. During early discussions there were ideas about canceling writeback and instead marking the folio dirty again. I assume there is a non-trivial solution space left unexplored for now. > > Maybe doing whack-a-mole results in less mess overall :-/ > Maybe :) I'm fine with the patch as is as well. >> As a big temporary kernel hack, we could add a >> AS_ANY_WAITING_UTTERLY_BROKEN and simply refuse to wait for writeback >> directly inside folio_wait_writeback() -- not arbitrarily skipping it in >> callers -- and possibly other places (readahead, not sure). That would >> restore the old behavior. > > No it wouldn't, since the old code had surrogate methods for waiting > on outstanding writes, which were called on fsync, etc. Yeah, I raised some "except" below, I assume there are more. No that I would want to go down that path :) -- Cheers David