Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when shrinking

[Yang Shi]

On 9/27/18 5:50 AM, Kirill A. Shutemov wrote:
> 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?

Aha, thanks for catching this. It can be moved inside down_write().

Will solve in the next version.

Thanks,
Yang

> >   	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