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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-66e4ed2c78esm1613351d50.19.2026.08.28.15.20.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Aug 2026 15:20:16 -0700 (PDT) Date: Fri, 28 Aug 2026 15:20:11 -0700 (PDT) From: Hugh Dickins To: Matthew Wilcox cc: Hugh Dickins , Kiryl Shutsemau , Andrew Morton , Ackerley Tng , Alexander Viro , Baolin Wang , Barry Song , Binbin Wu , Christian Brauner , Christoph Hellwig , Christoph Lameter , Claudio Imbrenda , David Hildenbrand , JP Kobryn , Jan Kara , Jens Axboe , Johannes Weiner , Kairui Song , Lance Yang , Leonardo Bras , Lorenzo Stoakes , Marcelo Tosatti , Mel Gorman , Miaohe Lin , Michal Hocko , Minchan Kim , Muchun Song , Oscar Salvador , Peter Zijlstra , Qi Zheng , Rik van Riel , Sebastian Andrzej Siewior , Shakeel Butt , Suren Baghdasaryan , Vlastimil Babka , Yang Shi , Yu Zhao , Zach O'Keefe , Zi Yan , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 04/25] mm/fbatch: lru bit set, no extra ref, while folio on per-cpu fbatch In-Reply-To: Message-ID: References: <14a16945-529b-8bc0-ab38-3ea97e54e223@google.com> <3b8d9cc4-d6f9-6bd6-f774-34cfca9c48c9@google.com> 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 On Fri, 28 Aug 2026, Matthew Wilcox wrote: > On Fri, Aug 28, 2026 at 01:04:34AM -0700, Hugh Dickins wrote: > > On Thu, 27 Aug 2026, Kiryl Shutsemau wrote: > > > On Mon, Aug 24, 2026 at 07:01:20AM -0700, Hugh Dickins wrote: > > > > Treat folios on a per-cpu fbatch as if they were already on the lruvec: > > > > with PG_lru set, without holding an extra reference. This will enable > > > > the removal of most lru_add_drain() and lru_add_drain_all() calls soon. > > > > > > > > Recognize such a folio by 0x02 set in the folio->lru.next pointer by > > > > folio_add_lru(). > > > > > > Hm. pfmemalloc (__GFP_MEMALLOC) thingy already claims the bit. Is it > > > safe because such memory is never on LRU? > > > > > > Are pfmemalloc and PG_lru mutually exclusive? > > > > > > Do we want to be explicit about this? Like, folio/page_is_pfmemalloc() > > > shouldn't return true for PG_lru folios/pages or something. > > > > Gosh, thanks so much for pointing that out: I was completely ignorant > > of the the pfmemalloc use, and a bit (bit 1!) shocked to learn of it > > (why wouldn't they just reuse a pageflag, I wonder? but doesn't matter). > > The idea is that it is used temporarily to communicate from the > page allocator to the caller "This only succeeded because of > PF_MEMALLOC". Making it a page flag would require all callers be > aware of it -- most simply do not care. PF_MEMALLOC was set on > their behalf and there is nothing they can usefully do with this > information. > > So we want to communicate it in a way that allows the unaware caller to > discard the information, and I chose bit 1 of page->lru.next. We > used to use page->index == -1UL (which was also naturally overwritten by > the unaware caller), but we needed to have it be part of PP_SIGNATURE > and that needed to be not part of page->index ... > > Commit dc8cf7550a70 if you want to read more about it. Thanks for explaining, I had looked at some of the other commits implicated, but not that one. So, it's a special snowflake which can melt away before it interferes with anyone not interested (which a page flag would not): yes, that makes some sense. > > We do use a page flag in slab; we check the pfmalloc bit and move it > into a page flag (SL_pfmemalloc). But we don't use it for folios. > > While there is a folio_is_pfmemalloc(), I think that was a mistake > and it should now be deleted. It hasn't been used since slab was > converted away from folios last November. > > > Anyway, as you've rightly guessed, it's not a problem at all: these > > mm/folio.c and mm/mlock.c per-cpu fbatches are entirely for folios; > > and if any pfmemalloced page ever get used for a folio (dunno) and > > put on an fbatch for LRU, then of course its use of lru.next is > > immediately overwritten (first by what this patch writes in lru_next, > > then later by the lru.next pointer for whatever LRU it goes on to - > > just as before this patch). > > > > If you were to tell me that some subsystem uses PG_lru for some > > other purpose, then I would have to get more worried; but we can > > be fairly sure that's not so, since mm/compaction.c for one relies > > on konwing it's free to play with PG_lru folios. > > > > Whether a folio is ever allocated with __GFP_MEMALLOC, I'm not > > certain (haven't looked), but there is no need to exclude that: > > it simply would not retain that page_is_pfmemalloc() info across > > folio_add_lru(). > > That's the correct thinking. > > How about this patch? Oh, when I said "This certainly deserves a comment somewhere", I meant an acknowledgement somewhere in my series, not a complaint that it had not been already commented. Once Kiryl pointed me, I found the comment in page_is_pfmemalloc() good enough: the problem is not the wording of the comment, but knowing where and when to look for such a comment. We have many header files, and understandably no registry of low bits in pointers. Wherever you put a good comment, I'd have missed it. So, I was okay with the original, but your longer explanation below looks fine, and the removal of folio_is_pfmemalloc() good, but best of all is the comment line (nit: wants a full stop) you add in mm_types.h. Thanks, Hugh > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index dd09c438fa23..fcefffb52d3e 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -3122,36 +3122,25 @@ static inline void *folio_address(const struct folio *folio) > return page_address(&folio->page); > } > > -/* > - * Return true only if the page has been allocated with > - * ALLOC_NO_WATERMARKS and the low watermark was not > - * met implying that the system is under some pressure. > +/** > + * page_is_pfmemalloc - Page allocation should have failed > + * @page: The just-allocated page > + * > + * Usually the page allocator keeps some memory in reserve. If > + * __GFP_MEMALLOC is used, the page allocator can dip into those > + * reserves. The caller can find out if the allocation came from > + * the reservers by calling this function. > + * > + * If the caller does not care, it can simply use the page as > + * normal. The field that the information is stored in is usually > + * overwritten by most uses of a page. Nobody should call this for > + * a page they did not allocate as it can easily have false positives. > */ > static inline bool page_is_pfmemalloc(const struct page *page) > { > - /* > - * lru.next has bit 1 set if the page is allocated from the > - * pfmemalloc reserves. Callers may simply overwrite it if > - * they do not need to preserve that information. > - */ > return (uintptr_t)page->lru.next & BIT(1); > } > > -/* > - * Return true only if the folio has been allocated with > - * ALLOC_NO_WATERMARKS and the low watermark was not > - * met implying that the system is under some pressure. > - */ > -static inline bool folio_is_pfmemalloc(const struct folio *folio) > -{ > - /* > - * lru.next has bit 1 set if the page is allocated from the > - * pfmemalloc reserves. Callers may simply overwrite it if > - * they do not need to preserve that information. > - */ > - return (uintptr_t)folio->lru.next & BIT(1); > -} > - > /* > * Only to be called by the page allocator on a freshly allocated > * page. > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 09cb1e18bc16..9bcfc4213b91 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -85,6 +85,7 @@ struct page { > * WARNING: bit 0 of the first word is used for PageTail(). That > * means the other users of this union MUST NOT use the bit to > * avoid collision and false-positive PageTail(). > + * Bit 1 of the first word is used by page_is_pfmemalloc() > */ > union { > struct { /* Page cache and anonymous pages */