Re: [PATCH V2 7/7] mm: add a separate RSS for MADV_FREE pages

[Michal Hocko]

On Fri 10-02-17 10:01:02, Shaohua Li wrote:
> On Fri, Feb 10, 2017 at 02:35:05PM +0100, Michal Hocko wrote:
> > On Fri 03-02-17 15:33:23, Shaohua Li wrote:
> > > Add a separate RSS for MADV_FREE pages. The pages are charged into
> > > MM_ANONPAGES (because they are mapped anon pages) and also charged into
> > > the MM_LAZYFREEPAGES. /proc/pid/statm will have an extra field to
> > > display the RSS, which userspace can use to determine the RSS excluding
> > > MADV_FREE pages.
> > > 
> > > The basic idea is to increment the RSS in madvise and decrement in unmap
> > > or page reclaim. There is one limitation. If a page is shared by two
> > > processes, since madvise only has mm cotext of current process, it isn't
> > > convenient to charge the RSS for both processes. So we don't charge the
> > > RSS if the mapcount isn't 1. On the other hand, fork can make a
> > > MADV_FREE page shared by two processes. To make things consistent, we
> > > uncharge the RSS from the source mm in fork.
> > > 
> > > A new flag is added to indicate if a page is accounted into the RSS. We
> > > can't use SwapBacked flag to do the determination because we can't
> > > guarantee the page has SwapBacked flag cleared in madvise. We are
> > > reusing mappedtodisk flag which should not be set for Anon pages.
> > > 
> > > There are a couple of other places we need to uncharge the RSS,
> > > activate_page and mark_page_accessed. activate_page is used by swap,
> > > where MADV_FREE pages are already not in lazyfree state before going
> > > into swap. mark_page_accessed is mainly used for file pages, but there
> > > are several places it's used by anonymous pages. I fixed gup, but not
> > > some gpu drivers and kvm. If the drivers use MADV_FREE, we might have
> > > inprecise RSS accounting.
> > > 
> > > Please note, the accounting is never going to be precise. MADV_FREE page
> > > could be written by userspace without notification to the kernel. The
> > > page can't be reclaimed like other clean lazyfree pages. The page isn't
> > > real lazyfree page. But since kernel isn't aware of this, the page is
> > > still accounted as lazyfree, thus the accounting could be incorrect.
> > 
> > This is all quite complex and as you say unprecise already. From the
> > description it is not even clear why do we need it at all. Why is
> > /proc/<pid>/smaps insufficient? I am also not fun of a new page flag -
> > even though you managed to recycle an existing one which is a plus.
> 
> We have monitor app running in the system to check other apps' RSS and kill
> them if RSS is abnormal. Checking /proc/pid/smaps is too complicated and slow,
> don't think we can go that way.

Could you be more specific about why "slow" matters?

-- 
Michal Hocko
SUSE Labs