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    SubjectRe: [PATCH V6 3/6] mm: Introduce VM_LOCKONFAULT
    On Thu, Aug 06, 2015 at 05:33:54PM +0200, Vlastimil Babka wrote:
    > On 07/29/2015 05:42 PM, Eric B Munson wrote:
    > >The cost of faulting in all memory to be locked can be very high when
    > >working with large mappings. If only portions of the mapping will be
    > >used this can incur a high penalty for locking.
    > >
    > >For the example of a large file, this is the usage pattern for a large
    > >statical language model (probably applies to other statical or graphical
    > >models as well). For the security example, any application transacting
    > >in data that cannot be swapped out (credit card data, medical records,
    > >etc).
    > >
    > >This patch introduces the ability to request that pages are not
    > >pre-faulted, but are placed on the unevictable LRU when they are finally
    > >faulted in. The VM_LOCKONFAULT flag will be used together with
    > >VM_LOCKED and has no effect when set without VM_LOCKED. Setting the
    > >VM_LOCKONFAULT flag for a VMA will cause pages faulted into that VMA to
    > >be added to the unevictable LRU when they are faulted or if they are
    > >already present, but will not cause any missing pages to be faulted in.
    > >
    > >Exposing this new lock state means that we cannot overload the meaning
    > >of the FOLL_POPULATE flag any longer. Prior to this patch it was used
    > >to mean that the VMA for a fault was locked. This means we need the
    > >new FOLL_MLOCK flag to communicate the locked state of a VMA.
    > >FOLL_POPULATE will now only control if the VMA should be populated and
    > >in the case of VM_LOCKONFAULT, it will not be set.
    > >
    > >Signed-off-by: Eric B Munson <emunson@akamai.com>
    > >Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > >Cc: Michal Hocko <mhocko@suse.cz>
    > >Cc: Vlastimil Babka <vbabka@suse.cz>
    > >Cc: Jonathan Corbet <corbet@lwn.net>
    > >Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
    > >Cc: linux-kernel@vger.kernel.org
    > >Cc: dri-devel@lists.freedesktop.org
    > >Cc: linux-mm@kvack.org
    > >Cc: linux-api@vger.kernel.org
    > >---
    > > drivers/gpu/drm/drm_vm.c | 8 +++++++-
    > > fs/proc/task_mmu.c | 1 +
    > > include/linux/mm.h | 2 ++
    > > kernel/fork.c | 2 +-
    > > mm/debug.c | 1 +
    > > mm/gup.c | 10 ++++++++--
    > > mm/huge_memory.c | 2 +-
    > > mm/hugetlb.c | 4 ++--
    > > mm/mlock.c | 2 +-
    > > mm/mmap.c | 2 +-
    > > mm/rmap.c | 4 ++--
    > > 11 files changed, 27 insertions(+), 11 deletions(-)
    > >
    > >diff --git a/drivers/gpu/drm/drm_vm.c b/drivers/gpu/drm/drm_vm.c
    > >index aab49ee..103a5f6 100644
    > >--- a/drivers/gpu/drm/drm_vm.c
    > >+++ b/drivers/gpu/drm/drm_vm.c
    > >@@ -699,9 +699,15 @@ int drm_vma_info(struct seq_file *m, void *data)
    > > (void *)(unsigned long)virt_to_phys(high_memory));
    > >
    > > list_for_each_entry(pt, &dev->vmalist, head) {
    > >+ char lock_flag = '-';
    > >+
    > > vma = pt->vma;
    > > if (!vma)
    > > continue;
    > >+ if (vma->vm_flags & VM_LOCKONFAULT)
    > >+ lock_flag = 'f';
    > >+ else if (vma->vm_flags & VM_LOCKED)
    > >+ lock_flag = 'l';
    > > seq_printf(m,
    > > "\n%5d 0x%pK-0x%pK %c%c%c%c%c%c 0x%08lx000",
    > > pt->pid,
    > >@@ -710,7 +716,7 @@ int drm_vma_info(struct seq_file *m, void *data)
    > > vma->vm_flags & VM_WRITE ? 'w' : '-',
    > > vma->vm_flags & VM_EXEC ? 'x' : '-',
    > > vma->vm_flags & VM_MAYSHARE ? 's' : 'p',
    > >- vma->vm_flags & VM_LOCKED ? 'l' : '-',
    > >+ lock_flag,
    > > vma->vm_flags & VM_IO ? 'i' : '-',
    > > vma->vm_pgoff);
    > >
    > >diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    > >index ca1e091..38d69fc 100644
    > >--- a/fs/proc/task_mmu.c
    > >+++ b/fs/proc/task_mmu.c
    > >@@ -579,6 +579,7 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
    >
    > This function has the following comment:
    >
    > Don't forget to update Documentation/ on changes.
    >
    > [...]
    >
    > >--- a/mm/gup.c
    > >+++ b/mm/gup.c
    > >@@ -92,7 +92,7 @@ retry:
    > > */
    > > mark_page_accessed(page);
    > > }
    > >- if ((flags & FOLL_POPULATE) && (vma->vm_flags & VM_LOCKED)) {
    > >+ if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
    > > /*
    > > * The preliminary mapping check is mainly to avoid the
    > > * pointless overhead of lock_page on the ZERO_PAGE
    > >@@ -265,6 +265,9 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
    > > unsigned int fault_flags = 0;
    > > int ret;
    > >
    > >+ /* mlock all present pages, but do not fault in new pages */
    > >+ if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
    > >+ return -ENOENT;
    > > /* For mm_populate(), just skip the stack guard page. */
    > > if ((*flags & FOLL_POPULATE) &&
    > > (stack_guard_page_start(vma, address) ||
    > >@@ -850,7 +853,10 @@ long populate_vma_page_range(struct vm_area_struct *vma,
    > > VM_BUG_ON_VMA(end > vma->vm_end, vma);
    > > VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
    > >
    > >- gup_flags = FOLL_TOUCH | FOLL_POPULATE;
    > >+ gup_flags = FOLL_TOUCH | FOLL_MLOCK;
    > >+ if ((vma->vm_flags & (VM_LOCKED | VM_LOCKONFAULT)) == VM_LOCKED)
    > >+ gup_flags |= FOLL_POPULATE;
    > >+
    > > /*
    > > * We want to touch writable mappings with a write fault in order
    > > * to break COW, except for shared mappings because these don't COW
    >
    > I think this might be breaking the populate part of mmap(MAP_POPULATE &
    > ~MAP_LOCKED) case, if I follow the execution correctly (it's far from
    > simple...)
    >
    > SYSCALL_DEFINE6(mmap_pgoff... with MAP_POPULATE
    > vm_mmap_pgoff(..., MAP_POPULATE...)
    > do_mmap_pgoff(...MAP_POPULATE... &populate) -> populate == TRUE
    > mm_populate()
    > __mm_populate()
    > populate_vma_page_range()
    >
    > Previously, this path would have FOLL_POPULATE in gup_flags and continue
    > with __get_user_pages() and faultin_page() (actually regardless of
    > FOLL_POPULATE) which would fault in the pages.
    >
    > After your patch, populate_vma_page_range() will set FOLL_MLOCK, but since
    > VM_LOCKED is not set, FOLL_POPULATE won't be set either.
    > Then faultin_page() will return on the new check:
    >
    > flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK

    Good catch!

    I guess it should be something like:

    gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK;
    if (vma->vm_flags & VM_LOCKONFAULT)
    gup_flags &= ~FOLL_POPULATE;

    --
    Kirill A. Shutemov


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