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SubjectRe: [PATCH] mm: call pte_unmap() against a proper pte (Re: [PATCH 7/9] swap_info: swap count continuations)
On Fri, 16 Oct 2009 15:30:56 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Hi.
>
> > @@ -645,6 +648,7 @@ static int copy_pte_range(struct mm_stru
> > spinlock_t *src_ptl, *dst_ptl;
> > int progress = 0;
> > int rss[2];
> > + swp_entry_t entry = (swp_entry_t){0};
> >
> > again:
> > rss[1] = rss[0] = 0;
> > @@ -671,7 +675,10 @@ again:
> > progress++;
> > continue;
> > }
> > - copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
> > + entry.val = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte,
> > + vma, addr, rss);
> > + if (entry.val)
> > + break;
> > progress += 8;
> > } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
> >
> It isn't the fault of only this patch, but I think breaking the loop without incrementing
> dst_pte(and src_pte) would be bad behavior because we do unmap_pte(dst_pte - 1) later.
> (current copy_pte_range() already does it though... and this is only problematic
> when we break the first loop, IIUC.)
>

oh, yes. nice catch!

> > @@ -681,6 +688,12 @@ again:
> > add_mm_rss(dst_mm, rss[0], rss[1]);
> > pte_unmap_unlock(dst_pte - 1, dst_ptl);
> > cond_resched();
> > +
> > + if (entry.val) {
> > + if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> > + return -ENOMEM;
> > + progress = 0;
> > + }
> > if (addr != end)
> > goto again;
> > return 0;
>
> I've searched other places where we break a similar loop and do pte_unmap(pte - 1).
> Current copy_pte_range() and apply_to_pte_range() has the same problem.
>

> How about a patch like this ?
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
>
> There are some places where we do like:
>
> pte = pte_map();
> do {
> (do break in some conditions)
> } while (pte++, ...);
> pte_unmap(pte - 1);
>
> But if the loop breaks at the first loop, pte_unmap() unmaps invalid pte.
>
> This patch is a fix for this problem.
>
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

seems correct.

Reviewd-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

> ---
> mm/memory.c | 11 +++++++----
> 1 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 72a2494..492de38 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -641,6 +641,7 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> pmd_t *dst_pmd, pmd_t *src_pmd, struct vm_area_struct *vma,
> unsigned long addr, unsigned long end)
> {
> + pte_t *orig_src_pte, *orig_dst_pte;
> pte_t *src_pte, *dst_pte;
> spinlock_t *src_ptl, *dst_ptl;
> int progress = 0;
> @@ -654,6 +655,8 @@ again:
> src_pte = pte_offset_map_nested(src_pmd, addr);
> src_ptl = pte_lockptr(src_mm, src_pmd);
> spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
> + orig_src_pte = src_pte;
> + orig_dst_pte = dst_pte;
> arch_enter_lazy_mmu_mode();
>
> do {
> @@ -677,9 +680,9 @@ again:
>
> arch_leave_lazy_mmu_mode();
> spin_unlock(src_ptl);
> - pte_unmap_nested(src_pte - 1);
> + pte_unmap_nested(orig_src_pte);
> add_mm_rss(dst_mm, rss[0], rss[1]);
> - pte_unmap_unlock(dst_pte - 1, dst_ptl);
> + pte_unmap_unlock(orig_dst_pte, dst_ptl);
> cond_resched();
> if (addr != end)
> goto again;
> @@ -1822,10 +1825,10 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
> token = pmd_pgtable(*pmd);
>
> do {
> - err = fn(pte, token, addr, data);
> + err = fn(pte++, token, addr, data);
> if (err)
> break;
> - } while (pte++, addr += PAGE_SIZE, addr != end);
> + } while (addr += PAGE_SIZE, addr != end);
>
> arch_leave_lazy_mmu_mode();
>
>



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