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Wed, 24 May 2023 14:59:29 -0700 (PDT) MIME-Version: 1.0 References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com> <68dddbbb-bc3c-367d-798a-cbe2a69664@google.com> In-Reply-To: <68dddbbb-bc3c-367d-798a-cbe2a69664@google.com> From: Yang Shi Date: Wed, 24 May 2023 14:59:17 -0700 Message-ID: Subject: Re: [PATCH 27/31] mm/khugepaged: allow pte_offset_map[_lock]() to fail To: Hugh Dickins Cc: Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Mel Gorman , Peter Xu , Peter Zijlstra , Will Deacon , Yu Zhao , Alistair Popple , Ralph Campbell , Ira Weiny , Steven Price , SeongJae Park , Naoya Horiguchi , Christophe Leroy , Zack Rusin , Jason Gunthorpe , Axel Rasmussen , Anshuman Khandual , Pasha Tatashin , Miaohe Lin , Minchan Kim , Christoph Hellwig , Song Liu , Thomas Hellstrom , linux-kernel@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 8D7B9100019 X-Stat-Signature: pebcncbxobk6dzbnsmg75obj95cuf89k X-Rspam-User: X-HE-Tag: 1684965570-18937 X-HE-Meta: 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 qq5cPlXC 0oGBMfyNnsILdGROdL/nbuaLBrChwsNEws8o98wdXiknQTZ1C2E/X94zuTyVMoGgMSiukAh1L8WzqiXuPE8+j9mvCIijNXwVyZJQZrYV7PskaFdBym4fqDx4nG2ZyYH1Gvo5aI/GluwEV4O+5rmnO2WKfTowGJeHJ2ds1VQ3wRF0PdnXK0mHSE9HXS3YUdrNwe4F35ZVFzG6AO50bxSs6CjzNJt4WT3Mwm71GAXCEDoWm/choQrHmtC3Vy42WG6GI5jboA/Ql2NoQDLpzicyWZpaeIApSsFSiITJOaydN8jmoF6GlJDOBA41QBHssYMf71ENP5x0RtcK6uU9tJ8elynl4yKUuNypMXityW1L8P/jlU10cndMA/NsGEv7wtS0+izKIA/K0VihKzyX7Jh5gW32xflu8GXCt7K2MhYb5UicxCho= 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 Tue, May 23, 2023 at 9:44=E2=80=AFPM Hugh Dickins wro= te: > > On Mon, 22 May 2023, Yang Shi wrote: > > On Sun, May 21, 2023 at 10:24=E2=80=AFPM Hugh Dickins wrote: > > > > > > __collapse_huge_page_swapin(): don't drop the map after every pte, it > > > only has to be dropped by do_swap_page(); give up if pte_offset_map() > > > fails; trace_mm_collapse_huge_page_swapin() at the end, with result; > > > fix comment on returned result; fix vmf.pgoff, though it's not used. > > > > > > collapse_huge_page(): use pte_offset_map_lock() on the _pmd returned > > > from clearing; allow failure, but it should be impossible there. > > > hpage_collapse_scan_pmd() and collapse_pte_mapped_thp() allow for > > > pte_offset_map_lock() failure. > > > > > > Signed-off-by: Hugh Dickins > > > > Reviewed-by: Yang Shi > > Thanks. > > > > > A nit below: > > > > > --- > > > mm/khugepaged.c | 72 +++++++++++++++++++++++++++++++++--------------= -- > > > 1 file changed, 49 insertions(+), 23 deletions(-) > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index 732f9ac393fc..49cfa7cdfe93 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > ... > > > @@ -1029,24 +1040,29 @@ static int __collapse_huge_page_swapin(struct= mm_struct *mm, > > > * resulting in later failure. > > > */ > > > if (ret & VM_FAULT_RETRY) { > > > - trace_mm_collapse_huge_page_swapin(mm, swappe= d_in, referenced, 0); > > > /* Likely, but not guaranteed, that page lock= failed */ > > > - return SCAN_PAGE_LOCK; > > > + result =3D SCAN_PAGE_LOCK; > > > > With per-VMA lock, this may not be true anymore, at least not true > > until per-VMA lock supports swap fault. It may be better to have a > > more general failure code, for example, SCAN_FAIL. But anyway you > > don't have to change it in your patch, I can send a follow-up patch > > once this series is landed on mm-unstable. > > Interesting point (I've not tried to wrap my head around what differences > per-VMA locking would make to old likelihoods here), and thank you for > deferring a change on it - appreciated. > > Something to beware of, if you do choose to change it: mostly those > SCAN codes (I'm not a fan of them!) are only for a tracepoint somewhere, > but madvise_collapse() and madvise_collapse_errno() take some of them > more seriously than others - I think SCAN_PAGE_LOCK ends up as an > EAGAIN (rightly), but SCAN_FAIL as an EINVAL (depends). > > But maybe there are layers in between which do not propagate the result > code, I didn't check. All in all, not something I'd spend time on myself= . Thanks, Hugh. A second look shows do_swap_page() should not return VM_FAULT_RETRY due to per-VMA lock since it depends on FAULT_FLAG_VMA_LOCK flag, but it is actually not set in khugepaged path. Khugepaged just has FAULT_FLAG_ALLOW_RETRY flag set. So we don't have to change anything. > > Hugh