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Message-ID: <55C37E62.6020909@suse.cz>
Date: Thu, 6 Aug 2015 17:33:54 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Eric B Munson <emunson@...mai.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Michal Hocko <mhocko@...e.cz>, Jonathan Corbet <corbet@....net>,
"Kirill A. Shutemov" <kirill@...temov.name>,
linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linux-mm@...ck.org, linux-api@...r.kernel.org
Subject: Re: [PATCH V6 3/6] mm: Introduce VM_LOCKONFAULT
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@...mai.com>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: "Kirill A. Shutemov" <kirill@...temov.name>
> Cc: linux-kernel@...r.kernel.org
> Cc: dri-devel@...ts.freedesktop.org
> Cc: linux-mm@...ck.org
> Cc: linux-api@...r.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
--
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