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Tue, 31 Jan 2023 07:04:07 -0800 (PST) Message-ID: <028c959d-e52a-5d08-6ac6-004ecdb3e549@kernel.dk> Date: Tue, 31 Jan 2023 08:04:06 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list) Content-Language: en-US To: David Hildenbrand , David Howells Cc: John Hubbard , Al Viro , Christoph Hellwig , Matthew Wilcox , Jan Kara , Jason Gunthorpe , Logan Gunthorpe , Jeff Layton , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com> <3351099.1675077249@warthog.procyon.org.uk> <3520518.1675116740@warthog.procyon.org.uk> <3791872.1675172490@warthog.procyon.org.uk> <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com> <7f8f2d0f-4bf2-71aa-c356-c78c6b7fd071@redhat.com> From: Jens Axboe In-Reply-To: <7f8f2d0f-4bf2-71aa-c356-c78c6b7fd071@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 21279180029 X-Stat-Signature: 69nxfp38mpgfrdm7nhxk84squxp9twng X-Rspam-User: X-HE-Tag: 1675177448-838088 X-HE-Meta: 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 xnJcnjDY Be/8cd1355v/08BBic7xoara8qwg8Dffot+JfaHJrnST/U6Eri8dNEpoHTw/sf+68cdp4Jj1yxiTrYNOXN5n8TcqdKNOj5Hw4UaOHN6lBfc0VijCl97wtBmsCLTe6xZJYJfvFVXY2tG9dPfXpkqfybifRdj91QJHByqbgO5Q/6x7R/wIvryQHiiTTLftvo70ERiwQ11qg9aU1bCXsde52IzSbaQs5YlkWaW7GD4KXLQR5CMMelZrsub+RAtTovSZvGJwiTmRs3i4skmCfN8WdWU0WLbXfG0Y1vNY2vohN1Zb9g0jsdRodHtPkCDHbm7u0DfrqnVBfurSBD5LKVKUXUwd4YfAjRKgezH5ZU39Rlq8mLAc6iV1nD4/YDD3uQWi3QDosDmwUQMWS5xpy0bayP+zM3XdZdLSR+aTPwH8Yxb3HFKpWcLeYNGn13cSIttq9k/2KizACpzSPRJ3uahdMbjBR09i5zBjRdheF X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 1/31/23 8:02?AM, David Hildenbrand wrote: > On 31.01.23 15:50, Jens Axboe wrote: >> On 1/31/23 6:48?AM, David Hildenbrand wrote: >>> On 31.01.23 14:41, David Howells wrote: >>>> David Hildenbrand wrote: >>>> >>>>>>> percpu counters maybe - add them up at the point of viewing? >>>>>> They are percpu, see my last email. But for every 108 changes (on >>>>>> my system), they will do two atomic_long_adds(). So not very >>>>>> useful for anything but low frequency modifications. >>>>>> >>>>> >>>>> Can we just treat the whole acquired/released accounting as a debug mechanism >>>>> to detect missing releases and do it only for debug kernels? >>>>> >>>>> >>>>> The pcpu counter is an s8, so we have to flush on a regular basis and cannot >>>>> really defer it any longer ... but I'm curious if it would be of any help to >>>>> only have a single PINNED counter that goes into both directions (inc/dec on >>>>> pin/release), to reduce the flushing. >>>>> >>>>> Of course, once we pin/release more than ~108 pages in one go or we switch >>>>> CPUs frequently it won't be that much of a help ... >>>> >>>> What are the stats actually used for? Is it just debugging, or do we actually >>>> have users for them (control groups spring to mind)? >>> >>> As it's really just "how many pinning events" vs. "how many unpinning >>> events", I assume it's only for debugging. >>> >>> For example, if you pin the same page twice it would not get accounted >>> as "a single page is pinned". >> >> How about something like the below then? I can send it out as a real >> patch, will run a sanity check on it first but would be surprised if >> this doesn't fix it. >> >> >> diff --git a/mm/gup.c b/mm/gup.c >> index f45a3a5be53a..41abb16286ec 100644 >> --- a/mm/gup.c >> +++ b/mm/gup.c >> @@ -168,7 +168,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) >> */ >> smp_mb__after_atomic(); >> +#ifdef CONFIG_DEBUG_VM >> node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs); >> +#endif >> return folio; >> } >> @@ -180,7 +182,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) >> static void gup_put_folio(struct folio *folio, int refs, unsigned int flags) >> { >> if (flags & FOLL_PIN) { >> +#ifdef CONFIG_DEBUG_VM >> node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs); >> +#endif >> if (folio_test_large(folio)) >> atomic_sub(refs, folio_pincount_ptr(folio)); >> else >> @@ -236,8 +240,9 @@ int __must_check try_grab_page(struct page *page, unsigned int flags) >> } else { >> folio_ref_add(folio, GUP_PIN_COUNTING_BIAS); >> } >> - >> +#ifdef CONFIG_DEBUG_VM >> node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1); >> +#endif >> } >> return 0; >> > > We might want to hide the counters completely by defining them only > with CONFIG_DEBUG_VM. Are all of them debug aids only? If so, yes we should just have node_stat_* under CONFIG_DEBUG_VM. -- Jens Axboe