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Fri, 29 May 2026 09:25:26 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id JOuRDoZbGWrnTwAAD6G6ig (envelope-from ); Fri, 29 May 2026 09:25:26 +0000 Date: Fri, 29 May 2026 10:25:24 +0100 From: Pedro Falcato To: Kaitao Cheng Cc: dennis@kernel.org, tj@kernel.org, cl@gentwo.org, akpm@linux-foundation.org, mhocko@suse.com, vbabka@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev, Kaitao Cheng Subject: Re: [PATCH 1/2] mm/percpu: Preserve NOFS/NOIO scope during chunk create and populate Message-ID: References: <20260528132917.81123-1-kaitao.cheng@linux.dev> <20260528132917.81123-2-kaitao.cheng@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260528132917.81123-2-kaitao.cheng@linux.dev> X-Stat-Signature: 9kbrr1tzfuq1kxt7pysegyydeyk18fe5 X-Rspamd-Queue-Id: B58D880005 X-Rspamd-Server: rspam07 X-Rspam-User: X-HE-Tag: 1780046728-16250 X-HE-Meta: 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 +hBlQRn8 zVNaLmvlUQDi+VllYhiOaBTG3ZHOpf5an/+MKzLAWXvt8jU7BudV0nb/gYW6GMrZzo+ko90rdl/1Px/+W3wyump3GzfE6r0dn5y/bSOJI7ey59wJ1JSlC52XWrczFaozTzlxQkYlO51e7vKRH+nxhaAUJ9/LSac4E9UvE1oPTMcLqxErOe+f5ANyp9l7UdcoEX4yotet+MePxVgUhQkU6OxXRXa+cKfhyG6QBKzWbZA8jcPCb+3iv4/LtzPlnyi5OrSaX5uON5GpC3jU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, May 28, 2026 at 09:29:16PM +0800, Kaitao Cheng wrote: > From: Kaitao Cheng > > pcpu_alloc_noprof() derives pcpu_gfp from the caller supplied GFP mask and > passes it to the backing percpu allocators. This preserves GFP_NOFS and > GFP_NOIO for pcpu_alloc_pages() and for the initial pcpu_chunk allocation. > > However, the chunk creation and population slow paths also call helpers > which do not take a GFP mask and perform internal allocations with > GFP_KERNEL. For example, pcpu_create_chunk() calls pcpu_get_vm_areas(), > and population can allocate temporary metadata or page tables while mapping > backing pages. As a result, a caller which explicitly uses GFP_NOFS or > GFP_NOIO can still enter FS or IO reclaim while creating or populating a > percpu chunk. > > This is problematic for callers which use GFP_NOFS or GFP_NOIO because > they are already holding filesystem or IO-path locks. If free chunks are > exhausted, the percpu allocation can take pcpu_alloc_mutex and then enter > unconstrained reclaim from these internal allocations, defeating the > caller's allocation context and potentially recreating reclaim lock > dependencies. > > Wrap chunk creation and population in a scoped NOIO or NOFS context when > pcpu_gfp has the corresponding constraints. Leave ordinary GFP_KERNEL > allocations unchanged so they retain full reclaim capability. > > Fixes: 9a5b183941b5 ("mm, percpu: do not consider sleepable allocations atomic") I assume you _did not_ observe this in production? As in no reclaim path should be insane^W daring enough to do pcpu allocations? > Signed-off-by: Kaitao Cheng > --- > mm/percpu.c | 26 ++++++++++++++++++++++++++ > 1 file changed, 26 insertions(+) > > diff --git a/mm/percpu.c b/mm/percpu.c > index 71a85d7245c7..1bb38467390b 100644 > --- a/mm/percpu.c > +++ b/mm/percpu.c > @@ -1778,6 +1778,23 @@ static void pcpu_alloc_tag_free_hook(struct pcpu_chunk *chunk, int off, size_t s > } > #endif > > +static unsigned int pcpu_memalloc_scope_save(gfp_t gfp) > +{ > + if (!(gfp & __GFP_IO)) > + return memalloc_noio_save(); > + if (!(gfp & __GFP_FS)) > + return memalloc_nofs_save(); > + return 0; > +} > + > +static void pcpu_memalloc_scope_restore(gfp_t gfp, unsigned int flags) > +{ > + if (!(gfp & __GFP_IO)) > + memalloc_noio_restore(flags); > + else if (!(gfp & __GFP_FS)) > + memalloc_nofs_restore(flags); > +} I disagree with this. We already have gfp flags, they're already passed to pcpu_create_chunk() and pcpu_populate_chunk(). It's their job to respect the gfp flags and Do The Right Thing(tm). Can you fix the problematic places? It seems like it's mostly the vmalloc backend that's problematic. > + > /** > * pcpu_alloc - the percpu allocator > * @size: size of area to allocate in bytes > @@ -1901,7 +1918,12 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved, > > /* No space left. Create a new chunk. */ > if (list_empty(&pcpu_chunk_lists[pcpu_free_slot])) { > + unsigned int pcpu_scope; > + > + pcpu_scope = pcpu_memalloc_scope_save(pcpu_gfp); > chunk = pcpu_create_chunk(pcpu_gfp); > + pcpu_memalloc_scope_restore(pcpu_gfp, pcpu_scope); > + > if (!chunk) { > err = "failed to allocate new chunk"; > goto fail; > @@ -1931,9 +1953,13 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved, > page_end = PFN_UP(off + size); > > for_each_clear_bitrange_from(rs, re, chunk->populated, page_end) { > + unsigned int pcpu_scope; > + > WARN_ON(chunk->immutable); > > + pcpu_scope = pcpu_memalloc_scope_save(pcpu_gfp); > ret = pcpu_populate_chunk(chunk, rs, re, pcpu_gfp); > + pcpu_memalloc_scope_restore(pcpu_gfp, pcpu_scope); > > spin_lock_irqsave(&pcpu_lock, flags); > if (ret) { > -- > 2.50.1 (Apple Git-155) > -- Pedro