lkml.org 
[lkml]   [2015]   [Sep]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCHv10 37/36, RFC] thp: allow mlocked THP again
From
Date
On 09/03/2015 05:16 PM, Kirill A. Shutemov wrote:
> This patch brings back mlocked THP. Instead of forbidding mlocked pages
> altogether, we just avoid mlocking PTE-mapped THPs and munlock THPs on
> split_huge_pmd().
>
> This means PTE-mapped THPs will be on normal lru lists and will be
> split under memory pressure by vmscan. After the split vmscan will
> detect unevictable small pages and mlock them.

Yeah that sounds like a compromise that should work.

> This way we can void leaking mlocked pages into non-VM_LOCKED VMAs.

avoid

But mlocked page in non-mlocked VMA's is a normal thing for shared pages
when only one of the sharing mm's did mlock(), right? So this
description doesn't explain the whole issue. I admit I forgot the exact
details already :(

>
> Not-Yet-Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>
> I'm not yet 100% certain that this approch is correct. Review would be appriciated.
> More testing is required.
>
> ---
> mm/gup.c | 6 ++++--
> mm/huge_memory.c | 33 +++++++++++++++++++++++-------
> mm/memory.c | 3 +--
> mm/mlock.c | 61 +++++++++++++++++++++++++++++++++++++-------------------
> 4 files changed, 71 insertions(+), 32 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 70d65e4015a4..e95b0cb6ed81 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -143,6 +143,10 @@ retry:
> mark_page_accessed(page);
> }
> if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
> + /* Do not mlock pte-mapped THP */
> + if (PageTransCompound(page))
> + goto out;
> +
> /*
> * The preliminary mapping check is mainly to avoid the
> * pointless overhead of lock_page on the ZERO_PAGE
> @@ -920,8 +924,6 @@ long populate_vma_page_range(struct vm_area_struct *vma,
> gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
> if (vma->vm_flags & VM_LOCKONFAULT)
> gup_flags &= ~FOLL_POPULATE;
> - if (vma->vm_flags & VM_LOCKED)
> - gup_flags |= FOLL_SPLIT;
> /*
> * We want to touch writable mappings with a write fault in order
> * to break COW, except for shared mappings because these don't COW
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2cc99f9096a8..d714de02473b 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -846,8 +846,6 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>
> if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
> return VM_FAULT_FALLBACK;
> - if (vma->vm_flags & VM_LOCKED)
> - return VM_FAULT_FALLBACK;
> if (unlikely(anon_vma_prepare(vma)))
> return VM_FAULT_OOM;
> if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
> @@ -1316,7 +1314,16 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> update_mmu_cache_pmd(vma, addr, pmd);
> }
> if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
> - if (page->mapping && trylock_page(page)) {
> + /*
> + * We don't mlock() pte-mapped THPs. This way we can avoid
> + * leaking mlocked pages into non-VM_LOCKED VMAs.
> + * In most cases the pmd is the only mapping of the page: we
> + * break COW for the mlock(). The only scenario when we have

I don't understand what's meant by "we break COW for the mlock()"?

> + * the page shared here is if we mlocking read-only mapping
> + * shared over fork(). We skip mlocking such pages.

Why do we skip them? There's no PTE mapping involved, just multiple PMD
mappings? Why are those a problem?




\
 
 \ /
  Last update: 2015-09-11 15:41    [W:0.121 / U:10.492 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site