Messages in this thread | | | Subject | Re: [PATCHv10 37/36, RFC] thp: allow mlocked THP again | From | Vlastimil Babka <> | Date | Fri, 11 Sep 2015 15:22:14 +0200 |
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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?
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