| Message ID | 20231219075538.414708-10-peterx@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/gup: Unify hugetlb, part 2 | expand |
On Tue, Dec 19, 2023 at 2:57 AM <peterx@redhat.com> wrote: > > From: Peter Xu <peterx@redhat.com> > > 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 <peterx@redhat.com> > --- > 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. > + 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. :) Feel free to add: Acked-by: James Houghton <jthoughton@google.com>
On Tue, Dec 19, 2023 at 11:28:54AM -0500, James Houghton wrote: > On Tue, Dec 19, 2023 at 2:57 AM <peterx@redhat.com> wrote: > > > > From: Peter Xu <peterx@redhat.com> > > > > 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 <peterx@redhat.com> > > --- > > 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 <jthoughton@google.com> Thanks,
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; + 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); - return follow_pmd_mask(vma, address, pud, flags, ctx); + return follow_pmd_mask(vma, address, pudp, flags, ctx); } static struct page *follow_p4d_mask(struct vm_area_struct *vma,