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Fri, 29 May 2026 09:38:10 +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 43EIJIJeGWqlXAAAD6G6ig (envelope-from ); Fri, 29 May 2026 09:38:10 +0000 Date: Fri, 29 May 2026 10:38:09 +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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Level: X-Spam-Score: -4.30 X-Spam-Flag: NO X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MISSING_XM_UA(0.00)[]; MIME_TRACE(0.00)[0:+]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCPT_COUNT_SEVEN(0.00)[11]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[pedro-suse.lan:mid,imap1.dmz-prg2.suse.org:helo] On Fri, May 29, 2026 at 10:25:28AM +0100, Pedro Falcato wrote: > 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? Oops, I mixed my issues up. This is purely a GFP flags issue. A quick "git grep alloc_percpu_gfp" shows that the vast majority (all?) callers are using some combination of GFP_KERNEL or GFP_ATOMIC + other GFP flags, but no NOFS or NOIO as far as I can see. So you probably did not observe this? > > > 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 -- Pedro