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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 00721157ae682-717c61b4d21sm4865197b3.77.2025.07.10.15.20.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Jul 2025 15:20:49 -0700 (PDT) Date: Thu, 10 Jul 2025 15:20:37 -0700 (PDT) From: Hugh Dickins To: Baolin Wang cc: Hugh Dickins , Brian Foster , linux-mm@kvack.org, Matthew Wilcox , Usama Arif Subject: Re: [PATCH] tmpfs: zero post-eof folio range on file extension In-Reply-To: <67f0461b-3359-41e7-a7cd-b059cbef4154@linux.alibaba.com> Message-ID: <097c0b07-1f43-51c3-3591-aaa2015226c2@google.com> References: <20250625184930.269727-1-bfoster@redhat.com> <297e44e9-1b58-d7c4-192c-9408204ab1e3@google.com> <67f0461b-3359-41e7-a7cd-b059cbef4154@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 952FB4001A X-Stat-Signature: 6earn4xzq3k51x7hc7ro65rtdkaghmz5 X-HE-Tag: 1752186052-419556 X-HE-Meta: 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 wp859fNL PPTlvcQekut18BP1VL68uPvclp5/JmprxDpL3PFas3BMo9g3tK5auqNYPRxrYxpAuZyoVn54qdPQ/CQiBjIYsm1cI9MTLkBsIqIpmQloQgQvx5o6318FrqC0siRl7eKGDug4qYpaJ5eDL8TA567abB1CuE2efBWEEoLBCbW23hUSkjNjZ7nM8/G6Zzs0Cbx5OujssPx0jgwxtEgBcN537LlPYZccumIHt4vDOXmRw1sVcKZk/SCdQ2ZfC+7hx2H4NACJC8nqOta2Q9uiBnsGRnSxJ/CHrDY08ax5kVRI81V2Yeoj8MyopqnFZAwh5RCUsYzsCVFgXP2w8yvxrLBxSKX8m+fPt3eiAkhprDQ9KeRUrDzc= 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 Thu, 10 Jul 2025, Baolin Wang wrote: > On 2025/7/9 15:57, Hugh Dickins wrote: ... > > > > The problem is with huge pages (or large folios) in shmem_writeout(): > > what goes in as a large folio may there have to be split into small > > pages; or it may be swapped out as one large folio, but fragmentation > > at swapin time demand that it be split into small pages when swapped in. > > Good point. > > > So, if there has been swapout since the large folio was modified beyond > > EOF, the folio that shmem_zero_eof() brings in does not guarantee what > > length needs to be zeroed. > > > > We could set that aside as a deficiency to be fixed later on: that > > would not be unreasonable, but I'm guessing that won't satisfy you. > > > > We could zero the maximum (the remainder of PMD size I believe) in > > shmem_zero_eof(): looping over small folios within the range, skipping > > !uptodate ones (but we do force them uptodate when swapping out, in > > order to keep the space reservation). TBH I've ignored that as a bad > > option, but it doesn't seem so bad to me now: ugly, but maybe not bad. > > However, IIUC, if the large folios are split in shmem_writeout(), and those > small folios which beyond EOF will be dropped and freed in > __split_unmapped_folio(), should we still consider them? You're absolutely right about the normal case, and thank you for making that point. Had I forgotten that when writing? Or was I already jumping ahead to the problem case? I don't recall, but was certainly wrong for not mentioning it. The abnormal case is when there's a "fallocend" beyond i_size (or beyond the small page extent spanning i_size) i.e. fallocate() has promised to keep pages allocated beyond EOF. In that case, __split_unmapped_folio() is keeping those pages. There could well be some optimization, involving fallocend, to avoid zeroing more than necessary; but I wouldn't want to say what in a hurry, it's quite confusing! Hugh