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V" , Andrew Morton , Jason Gunthorpe , Andrea Arcangeli , Axel Rasmussen Subject: Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask() Message-ID: References: <20231219075538.414708-1-peterx@redhat.com> <20231219075538.414708-10-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: DFD17100007 X-Stat-Signature: fmzykpq4boyusr1xcqkseohebb9n173o X-Rspam-User: X-HE-Tag: 1703036610-418888 X-HE-Meta: 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 ZyKIkc9Q i49iACMyL6XTbEGZp6tmiDJHdtEgRX7ZZBi/Y9PZFSM2QUEXG6lrCUrBVOHYkB8i1lXZ9ZwUI6aVLYliQ0Oz/01vqq3URKGysydY5xxLsynqt+A25ce7qHrH1IWYVr2GWnOexsuL2L+25IiaaNoH/wmFLUkSMnPgP6zjRhMbg+cnLk/slaPXpOhHeYWIbabFbLEIRp/YaL2xb+xGKbFoQz5N4aO5OTBatxn4hxBXdsMiF6ehs7ARxy7JvzdRT6paseFasb5oBVqmUosxHxur8BzyU082G4mc5Pncz/SQ+tQjRzpFDyS+/cSAa8HLzymZ0xfyzl9j29ZXhwic1f44u7isNwIlyZxApHcHO4E8YD4rDmw7CpP0zxbZowtJj0C04VNUMWljXdbXRX/2qBDuuNdRu3CwrGE92qpJkgKYDIqxyeFsMsxTFF7+jZtQOGVQkT2wLPf57sp9utfw= 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 11:28:54AM -0500, James Houghton wrote: > On Tue, Dec 19, 2023 at 2:57 AM wrote: > > > > From: Peter Xu > > > > Introduce "pud_t pud" in the function, so the code won't dereference *pudp > > multiple time. Not only because that looks less straightforward, but also > > 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 = vma->vm_mm; > > > > - pud = pud_offset(p4dp, address); > > - if (pud_none(*pud)) > > + pudp = pud_offset(p4dp, address); > > + pud = *pudp; > > I think you might want a READ_ONCE() on this so that the compiler > doesn't actually read the pud multiple times. Makes sense. I probably only did the "split" part which Christoph requested, without thinking futher than that. :) > > > + if (pud_none(pud)) > > return no_page_table(vma, flags, address); > > - if (pud_devmap(*pud)) { > > - ptl = pud_lock(mm, pud); > > - page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap); > > + if (pud_devmap(pud)) { > > + ptl = pud_lock(mm, pudp); > > + page = follow_devmap_pud(vma, address, pudp, flags, &ctx->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. :) I think the assumption here is it is expected to be a directory entry when reaching here, and for a valid directory entry pud_present() should always return true (a side note: pud_present() may not mean "PRESENT bit set", see m68k's implementation for example). Yeah I added that in the next patch, my intention was to check !pud_present() for all cases without the need to take pgtable lock, though. > > Feel free to add: > > Acked-by: James Houghton Thanks, -- Peter Xu