On Thu, Sep 27, 2018 at 02:10:34AM +0800, Yang Shi wrote:
> brk might be used to shinrk memory mapping too other than munmap().
s/shinrk/shrink/
> So, it may hold write mmap_sem for long time when shrinking large
> mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in
> munmap") described.
>
> The brk() will not manipulate vmas anymore after __do_munmap() call for
> the mapping shrink use case. But, it may set mm->brk after
> __do_munmap(), which needs hold write mmap_sem.
>
> However, a simple trick can workaround this by setting mm->brk before
> __do_munmap(). Then restore the original value if __do_munmap() fails.
> With this trick, it is safe to downgrade to read mmap_sem.
>
> So, the same optimization, which downgrades mmap_sem to read for
> zapping pages, is also feasible and reasonable to this case.
>
> The period of holding exclusive mmap_sem for shrinking large mapping
> would be reduced significantly with this optimization.
>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> ---
> v2: Rephrase the commit per Michal
>
> mm/mmap.c | 40 ++++++++++++++++++++++++++++++----------
> 1 file changed, 30 insertions(+), 10 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 017bcfa..0d2fae1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -193,9 +193,11 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
> unsigned long retval;
> unsigned long newbrk, oldbrk;
> struct mm_struct *mm = current->mm;
> + unsigned long origbrk = mm->brk;
Is it safe to read mm->brk outside the lock?
> struct vm_area_struct *next;
> unsigned long min_brk;
> bool populate;
> + bool downgrade = false;
Again,
s/downgrade/downgraded/ ?
> LIST_HEAD(uf);
>
> if (down_write_killable(&mm->mmap_sem))
> @@ -229,14 +231,29 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
>
> newbrk = PAGE_ALIGN(brk);
> oldbrk = PAGE_ALIGN(mm->brk);
> - if (oldbrk == newbrk)
> - goto set_brk;
> + if (oldbrk == newbrk) {
> + mm->brk = brk;
> + goto success;
> + }
>
> - /* Always allow shrinking brk. */
> + /*
> + * Always allow shrinking brk.
> + * __do_munmap() may downgrade mmap_sem to read.
> + */
> if (brk <= mm->brk) {
> - if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf))
> - goto set_brk;
> - goto out;
> + /*
> + * mm->brk need to be protected by write mmap_sem, update it
> + * before downgrading mmap_sem.
> + * When __do_munmap fail, it will be restored from origbrk.
> + */
> + mm->brk = brk;
> + retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
> + if (retval < 0) {
> + mm->brk = origbrk;
> + goto out;
> + } else if (retval == 1)
> + downgrade = true;
> + goto success;
> }
>
> /* Check against existing mmap mappings. */
> @@ -247,18 +264,21 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
> /* Ok, looks good - let it rip. */
> if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
> goto out;
> -
> -set_brk:
> mm->brk = brk;
> +
> +success:
> populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
> - up_write(&mm->mmap_sem);
> + if (downgrade)
> + up_read(&mm->mmap_sem);
> + else
> + up_write(&mm->mmap_sem);
> userfaultfd_unmap_complete(mm, &uf);
> if (populate)
> mm_populate(oldbrk, newbrk - oldbrk);
> return brk;
>
> out:
> - retval = mm->brk;
> + retval = origbrk;
> up_write(&mm->mmap_sem);
> return retval;
> }
> --
> 1.8.3.1
>
--
Kirill A. Shutemov
