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,
__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:
/*
* XArray node allocations from PF_MEMALLOC contexts could
* completely exhaust the page allocator. __GFP_NOMEMALLOC
* 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
