On Mon, 31 Jan 2011, Dave Hansen wrote:
>
> Right now, if a mm_walk has either ->pte_entry or ->pmd_entry
> set, it will unconditionally split and transparent huge pages
> it runs in to. In practice, that means that anyone doing a
>
> cat /proc/$pid/smaps
>
> will unconditionally break down every huge page in the process
> and depend on khugepaged to re-collapse it later. This is
> fairly suboptimal.
>
> This patch changes that behavior. It teaches each ->pmd_entry
> handler (there are three) that they must break down the THPs
> themselves. Also, the _generic_ code will never break down
> a THP unless a ->pte_entry handler is actually set.
>
> This means that the ->pmd_entry handlers can now choose to
> deal with THPs without breaking them down.
>
>
No sign-off (goes for patches 3 and 4, as well)?
> ---
>
> linux-2.6.git-dave/fs/proc/task_mmu.c | 6 ++++++
> linux-2.6.git-dave/mm/pagewalk.c | 24 ++++++++++++++++++++----
> 2 files changed, 26 insertions(+), 4 deletions(-)
>
> diff -puN mm/pagewalk.c~pagewalk-dont-always-split-thp mm/pagewalk.c
> --- linux-2.6.git/mm/pagewalk.c~pagewalk-dont-always-split-thp 2011-01-27 10:57:02.309914973 -0800
> +++ linux-2.6.git-dave/mm/pagewalk.c 2011-01-27 10:57:02.317914965 -0800
> @@ -33,19 +33,35 @@ static int walk_pmd_range(pud_t *pud, un
>
> pmd = pmd_offset(pud, addr);
> do {
> + again:
checkpatch will warn about the indent.
> next = pmd_addr_end(addr, end);
> - split_huge_page_pmd(walk->mm, pmd);
> - if (pmd_none_or_clear_bad(pmd)) {
> + if (pmd_none(*pmd)) {
Not sure why this has been changed from pmd_none_or_clear_bad(), that's
been done even prior to THP.
> if (walk->pte_hole)
> err = walk->pte_hole(addr, next, walk);
> if (err)
> break;
> continue;
> }
> + /*
> + * This implies that each ->pmd_entry() handler
> + * needs to know about pmd_trans_huge() pmds
> + */
Probably needs to be documented somewhere for users of pagewalk?
> if (walk->pmd_entry)
> err = walk->pmd_entry(pmd, addr, next, walk);
> - if (!err && walk->pte_entry)
> - err = walk_pte_range(pmd, addr, next, walk);
> + if (err)
> + break;
> +
> + /*
> + * Check this here so we only break down trans_huge
> + * pages when we _need_ to
> + */
> + if (!walk->pte_entry)
> + continue;
> +
> + split_huge_page_pmd(walk->mm, pmd);
> + if (pmd_none_or_clear_bad(pmd))
> + goto again;
> + err = walk_pte_range(pmd, addr, next, walk);
> if (err)
> break;
> } while (pmd++, addr = next, addr != end);
> diff -puN fs/proc/task_mmu.c~pagewalk-dont-always-split-thp fs/proc/task_mmu.c
> --- linux-2.6.git/fs/proc/task_mmu.c~pagewalk-dont-always-split-thp 2011-01-27 10:57:02.313914969 -0800
> +++ linux-2.6.git-dave/fs/proc/task_mmu.c 2011-01-27 10:57:02.321914961 -0800
> @@ -343,6 +343,8 @@ static int smaps_pte_range(pmd_t *pmd, u
> struct page *page;
> int mapcount;
>
> + split_huge_page_pmd(walk->mm, pmd);
> +
> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> for (; addr != end; pte++, addr += PAGE_SIZE) {
> ptent = *pte;
> @@ -467,6 +469,8 @@ static int clear_refs_pte_range(pmd_t *p
> spinlock_t *ptl;
> struct page *page;
>
> + split_huge_page_pmd(walk->mm, pmd);
> +
> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> for (; addr != end; pte++, addr += PAGE_SIZE) {
> ptent = *pte;
> @@ -623,6 +627,8 @@ static int pagemap_pte_range(pmd_t *pmd,
> pte_t *pte;
> int err = 0;
>
> + split_huge_page_pmd(walk->mm, pmd);
> +
> /* find the first VMA at or above 'addr' */
> vma = find_vma(walk->mm, addr);
> for (; addr != end; addr += PAGE_SIZE) {
--
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