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Mon, 09 Nov 2026 02:07:20 -0800 (PST) Received: from [192.168.1.22] ([83.231.69.9]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4959783f1d6sm8829668f8f.46.2026.11.09.02.07.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Nov 2026 02:07:20 -0800 (PST) Message-ID: Date: Mon, 9 Nov 2026 11:07:17 +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 v2 2/5] mm: limit pagecache folio refcount when splicing into pipes To: "David Hildenbrand (Arm)" , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Peter Xu , Johannes Weiner , Song Liu , Rik van Riel , Miklos Szeredi , Alexander Viro , Christian Brauner , Hugh Dickins , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , Arnd Bergmann , Jason Gunthorpe , Kevin Tian , "Joerg Roedel (AMD)" , Robin Murphy , Steve Sistare , John Hubbard , Josef Bacik Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, fuse-devel@lists.linux.dev, linux-arch@vger.kernel.org, iommu@lists.linux.dev, stable@vger.kernel.org References: <20261107-refcount_overflow_stable-v2-0-c38f6809b106@kernel.org> <20261107-refcount_overflow_stable-v2-2-c38f6809b106@kernel.org> <8d23b1eb-72aa-479d-9486-dbfd86d8414c@kernel.org> <43fc45ff-8636-4961-a80a-29844c15d7a1@kernel.org> Content-Language: en-US From: "Jose A. Perez de Azpillaga" In-Reply-To: <43fc45ff-8636-4961-a80a-29844c15d7a1@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/9/26 10:49 AM, David Hildenbrand (Arm) wrote: > On 11/9/26 10:30, David Hildenbrand (Arm) wrote: >> On 11/9/26 09:57, Jose A. Perez de Azpillaga wrote: >>> On 11/7/26 11:23 PM, David Hildenbrand (Arm) wrote: >>>> Similar to mapping pagecache folios into page tables, let's reject >>>> additional references when splicing folios into a pipe if the refcount >>>> is at its limit, similar to how we handle it during GUP. >>>> >>>> This will prevent more cases of possible folio refcount overflows with >>>> large pagecache folios. >>>> >>>> Let's keep it simple and check in the two callers of >>>> splice_folio_into_pipe(). Return -EOVERFLOW if the limit prevents any >>>> progress, so do_splice_read() can fall back to copy_splice_read() when >>>> read_iter is available. This keeps sendfile() and other splice users >>>> working when ordinary reads would still succeed. >>>> >>>> Also fall back to copying in FUSE when taking another reference through >>>> fuse_ref_folio() would exceed the limit. >>>> >>>> Also check in try_get_page() to cover pipe-buffer duplication by tee() >>>> and partial pipe-to-pipe splicing. >>>> >>>> Fixes: 800d8c63b2e9 ("shmem: add huge pages support") >>>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") >>>> Fixes: 793917d997df ("mm/readahead: Add large folio readahead") >>>> Cc: stable@vger.kernel.org >>>> Signed-off-by: David Hildenbrand (Arm) >>>> --- >>>> fs/fuse/dev.c | 5 +++-- >>>> fs/splice.c | 8 +++++++- >>>> include/linux/mm.h | 9 ++++++--- >>>> mm/filemap.c | 11 ++++++++--- >>>> mm/shmem.c | 4 ++++ >>>> 5 files changed, 28 insertions(+), 9 deletions(-) >>>> >>>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c >>>> index 3641abb90d6bb..4f6b7b42bfd3e 100644 >>>> --- a/fs/fuse/dev.c >>>> +++ b/fs/fuse/dev.c >>>> @@ -1265,9 +1265,10 @@ int fuse_copy_folio(struct fuse_copy_state *cs, struct folio **foliop, >>>> if (cs->write && cs->pipebufs && folio) { >>>> /* >>>> * Can't control lifetime of pipe buffers, so always >>>> - * copy user pages. >>>> + * copy user pages. Also copy folios at their refcount limit. >>>> */ >>>> - if (cs->req->args->user_pages) { >>>> + if (cs->req->args->user_pages || >>>> + unlikely(folio_ref_count_over_limit(folio))) { >>>> err = fuse_copy_fill(cs); >>>> if (err) >>>> return err; >>>> diff --git a/fs/splice.c b/fs/splice.c >>>> index e6d43329ed3f0..df73a4e65118b 100644 >>>> --- a/fs/splice.c >>>> +++ b/fs/splice.c >>>> @@ -956,6 +956,7 @@ static ssize_t do_splice_read(struct file *in, loff_t *ppos, >>>> unsigned int flags) >>>> { >>>> unsigned int p_space; >>>> + ssize_t ret; >>>> >>>> if (unlikely(!(in->f_mode & FMODE_READ))) >>>> return -EBADF; >>>> @@ -977,7 +978,12 @@ static ssize_t do_splice_read(struct file *in, loff_t *ppos, >>>> */ >>>> if ((in->f_flags & O_DIRECT) || IS_DAX(in->f_mapping->host)) >>>> return copy_splice_read(in, ppos, pipe, len, flags); >>>> - return in->f_op->splice_read(in, ppos, pipe, len, flags); >>>> + ret = in->f_op->splice_read(in, ppos, pipe, len, flags); >>>> + >>>> + /* Fallback to copying when hitting folio refcount limits. */ >>>> + if (unlikely(ret == -EOVERFLOW) && in->f_op->read_iter) >>>> + ret = copy_splice_read(in, ppos, pipe, len, flags); >>> >>> here it can't tell where the -EOVERFLOW came from, if another file >>> returns -EOVERFLOW (i.e. tty_fops uses copy_splice_read, and >>> n_hdlc_tty_read() returns -EOVERFLOW after dropping the frame), it now >>> gets a second read. >> >> >> That's what claude told me during review, and when asked whether that actually >> happened in practice it said no ;) >> >> So is that actually a problem? > > Double checked, claude managed to confuse itself. > > n_hdlc_tty_read() can indeed return -EOVERFLOW. > > I should just have let splice keep failing, dealing with copy fallback starts to > get annoying. > > I guess switching to -ETOOMANYREFS should work (and might be a better fitting error) I agree with that :) -- cheers, jose a. p-a