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Tue, 19 Dec 2023 08:29:31 -0800 (PST) MIME-Version: 1.0 References: <20231219075538.414708-1-peterx@redhat.com> <20231219075538.414708-10-peterx@redhat.com> In-Reply-To: <20231219075538.414708-10-peterx@redhat.com> From: James Houghton Date: Tue, 19 Dec 2023 11:28:54 -0500 Message-ID: Subject: Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask() To: peterx@redhat.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , Christophe Leroy , Lorenzo Stoakes , David Hildenbrand , Vlastimil Babka , Mike Kravetz , Mike Rapoport , Christoph Hellwig , John Hubbard , Andrew Jones , linux-arm-kernel@lists.infradead.org, Michael Ellerman , "Kirill A . Shutemov" , linuxppc-dev@lists.ozlabs.org, Rik van Riel , linux-riscv@lists.infradead.org, Yang Shi , "Aneesh Kumar K . V" , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Axel Rasmussen Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: DC0034002D X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: c8hwt3rb11adry1yrb16sf656otgke49 X-HE-Tag: 1703003372-26392 X-HE-Meta: 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 Az6RMNHW wPNkN9RnesLZ53DK47Vf+e4c/zyJ9JFjnpLfHFzHhCKCfVl6EV15r+/IOx3ap0diETElHy3MpNYqBUQAiOZ/pyXRahoQinpWEkNyK8HJKjhLQTjc/YRfYman6bJpz2m2mXnWj5Fap0GJbMD8ZQQiBsYiNZvIS6LLp2BILydkY9ZhUXeFkbcitcwbeI6QCEp74Fe3TjBbVIP8MHBqaMRZIsk+7KSRyZUSHN1QHkd5Faul1QKIG2A0BkGEZ0lCbuP08e7DMA89DP52fHxteU251WUgrK2i2ON0lgdB4lCNVhE1T04WZ57rxAIXWC5ZVivPqlF8+ 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: List-Subscribe: List-Unsubscribe: On Tue, Dec 19, 2023 at 2:57=E2=80=AFAM wrote: > > From: Peter Xu > > Introduce "pud_t pud" in the function, so the code won't dereference *pud= p > multiple time. Not only because that looks less straightforward, but als= o > because if the dereference really happened, it's not clear whether there > can be race to see different *pudp values if it's being modified at the > same time. > > Signed-off-by: Peter Xu > --- > mm/gup.c | 17 +++++++++-------- > 1 file changed, 9 insertions(+), 8 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 6c0d82fa8cc7..97e87b7a15c3 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_= struct *vma, > unsigned int flags, > struct follow_page_context *ctx) > { > - pud_t *pud; > + pud_t *pudp, pud; > spinlock_t *ptl; > struct page *page; > struct mm_struct *mm =3D vma->vm_mm; > > - pud =3D pud_offset(p4dp, address); > - if (pud_none(*pud)) > + pudp =3D pud_offset(p4dp, address); > + pud =3D *pudp; I think you might want a READ_ONCE() on this so that the compiler doesn't actually read the pud multiple times. > + if (pud_none(pud)) > return no_page_table(vma, flags, address); > - if (pud_devmap(*pud)) { > - ptl =3D pud_lock(mm, pud); > - page =3D follow_devmap_pud(vma, address, pud, flags, &ctx= ->pgmap); > + if (pud_devmap(pud)) { > + ptl =3D pud_lock(mm, pudp); > + page =3D follow_devmap_pud(vma, address, pudp, flags, &ct= x->pgmap); > spin_unlock(ptl); > if (page) > return page; > return no_page_table(vma, flags, address); > } > - if (unlikely(pud_bad(*pud))) > + if (unlikely(pud_bad(pud))) > return no_page_table(vma, flags, address); Not your change, but reading this, it's not clear to me that `pud_present(*pudp)` (and non-leaf) would necessarily be true at this point -- like, I would prefer to see `!pud_present(pud)` instead of `pud_bad()`. Thank you for adding that in the next patch. :) Feel free to add: Acked-by: James Houghton