If swap-out is using filesystem operations (SWP_FS_OPS), then it is not
safe to enter the FS for reclaim.
So only down-grade the requirement for swap pages to __GFP_IO after
checking that SWP_FS_OPS are not being used.
This makes the calculation of "may_enter_fs" slightly more complex, so
move it into a separate function. with that done, there is little value
in maintaining the bool variable any more. So replace the
may_enter_fs variable with a may_enter_fs() function. This removes any
risk for the variable becoming out-of-date.
Signed-off-by: NeilBrown <[email protected]>
---
mm/swap.h | 8 ++++++++
mm/vmscan.c | 29 ++++++++++++++++++++---------
2 files changed, 28 insertions(+), 9 deletions(-)
diff --git a/mm/swap.h b/mm/swap.h
index 13e72a5023aa..5c676e55f288 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -47,6 +47,10 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
struct page *swapin_readahead(swp_entry_t entry, gfp_t flag,
struct vm_fault *vmf);
+static inline unsigned int page_swap_flags(struct page *page)
+{
+ return page_swap_info(page)->flags;
+}
#else /* CONFIG_SWAP */
static inline int swap_readpage(struct page *page, bool do_poll)
{
@@ -126,4 +130,8 @@ static inline void clear_shadow_from_swap_cache(int type, unsigned long begin,
{
}
+static inline unsigned int page_swap_flags(struct page *page)
+{
+ return 0;
+}
#endif /* CONFIG_SWAP */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5c734ffc6057..ad5026d06aa8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1506,6 +1506,22 @@ static unsigned int demote_page_list(struct list_head *demote_pages,
return nr_succeeded;
}
+static bool may_enter_fs(struct page *page, gfp_t gfp_mask)
+{
+ if (gfp_mask & __GFP_FS)
+ return true;
+ if (!PageSwapCache(page) || !(gfp_mask & __GFP_IO))
+ return false;
+ /*
+ * We can "enter_fs" for swap-cache with only __GFP_IO
+ * providing this isn't SWP_FS_OPS.
+ * ->flags can be updated non-atomicially (scan_swap_map_slots),
+ * but that will never affect SWP_FS_OPS, so the data_race
+ * is safe.
+ */
+ return !data_race(page_swap_flags(page) & SWP_FS_OPS);
+}
+
/*
* shrink_page_list() returns the number of reclaimed pages
*/
@@ -1531,7 +1547,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
struct address_space *mapping;
struct page *page;
enum page_references references = PAGEREF_RECLAIM;
- bool dirty, writeback, may_enter_fs;
+ bool dirty, writeback;
unsigned int nr_pages;
cond_resched();
@@ -1555,9 +1571,6 @@ static unsigned int shrink_page_list(struct list_head *page_list,
if (!sc->may_unmap && page_mapped(page))
goto keep_locked;
- may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
- (PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
-
/*
* The number of dirty pages determines if a node is marked
* reclaim_congested. kswapd will stall and start writing
@@ -1602,7 +1615,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
* not to fs). In this case mark the page for immediate
* reclaim and continue scanning.
*
- * Require may_enter_fs because we would wait on fs, which
+ * Require may_enter_fs() because we would wait on fs, which
* may not have submitted IO yet. And the loop driver might
* enter reclaim, and deadlock if it waits on a page for
* which it is needed to do the write (loop masks off
@@ -1634,7 +1647,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
/* Case 2 above */
} else if (writeback_throttling_sane(sc) ||
- !PageReclaim(page) || !may_enter_fs) {
+ !PageReclaim(page) || !may_enter_fs(page, sc->gfp_mask)) {
/*
* This is slightly racy - end_page_writeback()
* might have just cleared PageReclaim, then
@@ -1724,8 +1737,6 @@ static unsigned int shrink_page_list(struct list_head *page_list,
goto activate_locked_split;
}
- may_enter_fs = true;
-
/* Adding to swap updated mapping */
mapping = page_mapping(page);
}
@@ -1795,7 +1806,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
if (references == PAGEREF_RECLAIM_CLEAN)
goto keep_locked;
- if (!may_enter_fs)
+ if (!may_enter_fs(page, sc->gfp_mask))
goto keep_locked;
if (!sc->may_writepage)
goto keep_locked;
On Mon, Jan 24, 2022 at 02:48:32PM +1100, NeilBrown wrote:
> If swap-out is using filesystem operations (SWP_FS_OPS), then it is not
> safe to enter the FS for reclaim.
> So only down-grade the requirement for swap pages to __GFP_IO after
> checking that SWP_FS_OPS are not being used.
>
> This makes the calculation of "may_enter_fs" slightly more complex, so
> move it into a separate function. with that done, there is little value
> in maintaining the bool variable any more. So replace the
> may_enter_fs variable with a may_enter_fs() function. This removes any
> risk for the variable becoming out-of-date.
Looks good,
Reviewed-by: Christoph Hellwig <[email protected]>