Re: [PATCH] mm: shmem: use proper gfp flags for shmem_writepage()

[Yang Shi]

On 11/6/19 7:18 AM, Michal Hocko wrote:
> On Wed 06-11-19 06:02:31, Yang Shi wrote:
> > The shmem_writepage() uses GFP_ATOMIC to allocate swap cache.
> > GFP_ATOMIC used to mean __GFP_HIGH, but now it means __GFP_HIGH |
> > __GFP_ATOMIC | __GFP_KSWAPD_RECLAIM.  However, shmem_writepage() should
> > write out to swap only in response to memory pressure, so
> > __GFP_KSWAPD_RECLAIM looks useless since the caller may be kswapd itself
> > or in direct reclaim already.
> What kind of problem are you trying to fix here?

I didn't run into any visible problem. I just happened to find this 
inconsistency when I was looking into the other problem.

The add_to_swap() does:

int add_to_swap(struct page *page)
{
...
err = add_to_swap_cache(page, entry,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
...
}

Actually, shmem_writepage() does almost the same thing and both of them 
are called in reclaim context, so I didn't see why they should use 
different gfp flag. And, GFP_ATOMIC is also different from the old 
definition as I mentioned in the commit log.

> > In addition, XArray node allocations from PF_MEMALLOC contexts could
> > completely exhaust the page allocator, __GFP_NOMEMALLOC stops emergency
> > reserves from being allocated.
> I am not really familiar with XArray much, could you be more specific
> please?

It comes from the comments of add_to_swap(), says:

/*
ÂÂÂÂÂÂÂÂ * XArray node allocations from PF_MEMALLOC contexts could
ÂÂÂÂÂÂÂÂ * completely exhaust the page allocator. __GFP_NOMEMALLOC
ÂÂÂÂÂÂÂÂ * stops emergency reserves from being allocated.

And, it looks the original comment came from pre-git time, TBH I'm not 
quite sure about the specific problem which incurred this. I suspect it 
may be because PF_MEMALLOC context allows ALLOC_NO_WATERMARK.

> > Here just copy the gfp flags used by add_to_swap().
> >
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/shmem.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 220be9f..9691dec 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1369,7 +1369,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
> >   	if (list_empty(&info->swaplist))
> >   		list_add(&info->swaplist, &shmem_swaplist);
> >   
> > -	if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
> > +	if (add_to_swap_cache(page, swap,
> > +			__GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN) == 0) {
> >   		spin_lock_irq(&info->lock);
> >   		shmem_recalc_inode(inode);
> >   		info->swapped++;
> > --
> > 1.8.3.1