Re: [PATCH 1/2] mm/madvise: help MADV_PAGEOUT to find swap cache pages

[Minchan Kim]

On Mon, Mar 23, 2020 at 04:41:49PM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> tl;dr: MADV_PAGEOUT ignores unmapped swap cache pages.  Enable
> MADV_PAGEOUT to find and reclaim swap cache.
> 
> The long story:
> 
> Looking for another issue, I wrote a simple test which had two
> processes: a parent and a fork()'d child.  The parent reads a
> memory buffer shared by the fork() and the child calls
> madvise(MADV_PAGEOUT) on the same buffer.
> 
> The first call to MADV_PAGEOUT does what is expected: it pages
> the memory out and causes faults in the parent.  However, after
> that, it does not cause any faults in the parent.  MADV_PAGEOUT
> only works once!  This was a surprise.
> 
> The PTEs in the shared buffer start out pte_present()==1 in
> both parent and child.  The first MADV_PAGEOUT operation replaces
> those with pte_present()==0 swap PTEs.  The parent process
> quickly faults and recreates pte_present()==1.  However, the
> child process (the one calling MADV_PAGEOUT) never touches the
> memory and has retained the non-present swap PTEs.
> 
> This situation could also happen in the case where a single
> process had some of its data placed in the swap cache but where
> the memory has not yet been reclaimed.
> 
> The MADV_PAGEOUT code has a pte_present()==0 check.  It will
> essentially ignore any pte_present()==0 pages.  This essentially
> makes unmapped swap cache immune from MADV_PAGEOUT, which is not
> very friendly behavior.
> 
> Enable MADV_PAGEOUT to find and reclaim swap cache.  Because
> swap cache is not pinned by holding the PTE lock, a reference
> must be held until the page is isolated, where a second
> reference is obtained.
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>