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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=mLh2i4Pn; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A6C4606A31D; Tue, 27 Oct 2026 10:43:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=mLh2i4Pn Received: by smtp.kernel.org (Postfix) with ESMTPSA id 087C01F01829; Tue, 27 Oct 2026 10:43:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793097831; bh=PmR+vZLuWo/MOqGjjBOdrO5nYefFznZmjJsKNuauJZY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=mLh2i4Pn549MOAtBAwtTl7y/1OVtrnhp4rO0PbVy/V6UmYdAI3NtxKLMphuOG7fLo wblUgc4S0AB7lxV1rH43/mDzSHF3W6Fynm7taDJXc1bX2QL4Nti6g4BSRnPmyk/DiO lNS7FaWQur6QkV/w+mv7H8Wm50v3XNiFOVIguHWqQ6oSLNB4R5+4WSZOHQkxrSuugc CMuIVKXwgrjt2zoc+cfKUOK3c473SQp01RiT5Df8tKOgu8lhdEcdUXMMrkXXCKHppD D83cximXZrxDk6WmbRv4v12unJ6ypdppHtcvqEWZQjSiy2ajbp1AztlNYQOhLWSW7u fD317OsWCcakQ== Message-ID: Date: Tue, 27 Oct 2026 11:12:41 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 11/16] mm/shmem: rescan after nonuniform folio splitting To: Zi Yan , Andrew Morton , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Johannes Weiner , Vladimir Davydov , Michal Hocko , Hugh Dickins , Baolin Wang , Lukas Czerner , Carlos Maiolino , Christian Brauner , Jan Kara , Daniel Verkamp , Jeff Xu , Muchun Song , Oscar Salvador , David Stevens , "Matthew Wilcox (Oracle)" , William Kucharski , Chris Li , Kairui Song , Nhat Pham , Miaohe Lin , Naoya Horiguchi Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org> <20261026-fixes_shmem-v1-11-703206d5fa0c@kernel.org> 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 10/27/26 03:32, Zi Yan wrote: > On Mon Oct 26, 2026 at 5:40 AM EDT, David Hildenbrand (Arm) wrote: >> The final scan in shmem_undo_range() restarts after a successful split >> only if the original folio is no longer large. A nonuniform split can >> leave that folio large, so the scan skips the newly created folios. >> Hole punching then leaves pages within the hole allocated even though >> they could be discarded. >> >> For example, a fault below the hole can allocate a 2 MiB folio after >> boundary handling. Punching a 1 MiB hole at offset 64 KiB splits the >> folio successfully but leaves the original folio at order 4. The scan >> continues past all 512 pages, leaving 2 MiB allocated instead of 1 MiB. >> The hole reads as zero, but the pages and their block charges remain. >> > > >> Restart when the original folio is still in the mapping after partial >> truncation, identified by checking folio->mapping. If the helper >> removed the whole folio, no rescan is needed. > > This paragraph is no longer needed after Zhang Yi's truncate fix, since > truncate_inode_partial_folio() returns true only if the folio is split. > The function returns false when the whole folio is truncated[1]. > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/tree/mm/truncate.c?h=mm-stable#n282 > >> >> Fixes: 7460b470a131f ("mm/truncate: use folio_split() in truncate operation") >> Cc: stable@vger.kernel.org >> Signed-off-by: David Hildenbrand (Arm) >> --- >> mm/shmem.c | 11 ++++------- >> 1 file changed, 4 insertions(+), 7 deletions(-) >> > > folio->mapping check is dropped and the comment is adjusted after the > patch is rebased on top of mm-stable because of the aforementioned > reason: > > > diff --git a/mm/shmem.c b/mm/shmem.c > index 7cf5a4d9d1ace..ddea31f44fb32 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -1477,18 +1477,12 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, > } else if (truncate_inode_partial_folio(folio, > lstart, lend, NULL, NULL)) { > /* > - * If we split a page, reset the loop so > - * that we pick up the new sub pages. > - * Otherwise the THP was entirely > - * dropped or the target range was > - * zeroed, so just continue the loop as > - * is. > + * The folio was split. Retry the scan > + * to find the split folios. > */ > - if (!folio_test_large(folio)) { > - folio_unlock(folio); > - index = start; > - break; > - } > + folio_unlock(folio); > + index = start; > + break; > } else { > /* > * Splitting failed, but the part in the > Yeah, that's straight forward. For this one I will keep Kiryl's ACK. Thanks all. -- Cheers, David