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Tue, 31 Jan 2023 07:15:26 -0800 (PST) Message-ID: <34f0dd4c-9d6a-bc66-f37d-329fb1620212@kernel.dk> Date: Tue, 31 Jan 2023 08:15:03 -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> <028c959d-e52a-5d08-6ac6-004ecdb3e549@kernel.dk> <0361aae6-59b2-1bbc-5530-a5be587b8a59@redhat.com> From: Jens Axboe In-Reply-To: <0361aae6-59b2-1bbc-5530-a5be587b8a59@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: fgdin5uptwdrpo8a175owfntmncnzqmq X-Rspamd-Queue-Id: 42F38C0014 X-HE-Tag: 1675178128-250263 X-HE-Meta: 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 aVcvWkCI 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 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 8:10 AM, David Hildenbrand wrote: > On 31.01.23 16:04, Jens Axboe wrote: >> 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. >> > > Rather only these 2. Smth like: Ah gotcha, that makes more sense to me. Will update the patch and send it out. -- Jens Axboe