If we exhaust the reserves in the page allocator when PF_MEMALLOC is set
then no longer give up but call into reclaim with PF_MEMALLOC set.
This is in essence a recursive call back into page reclaim with another
page flag (__GFP_NOMEMALLOC) set. The recursion is bounded since potential
allocations with __GFP_NOMEMALLOC set will not enter that branch again.
Allocation under PF_MEMALLOC will no longer run out outmemory if there
memory that is reclaimable without additional memory
allocations.
In order to make allocation-less reclaim working we need to avoid writing
pages out or swapping. So on entry to try_to_free_pages() we check for
__GFP_NOMEMALLOC. If it is set then sc.may_writepage and sc.mayswap are
switched off and we short circuit the writeout throttling.
The types of pages that can be reclaimed by a call to try_to_free_pages()
with the __GFP_NOMEMALLOC parameter are:
- Unmapped clean page cache pages.
- Mapped clean pages
- slab shrinking
We print a warning if we get into the special reclaim mode because
this means that the reserves are too low.
Changes
RFC->v1
- Allow slab shrinking in recursive reclaim (is protected by a
semaphore and already had to deal with allocs failing under
PF_MEMALLOC)
- Add printk to show that recursive reclaim is being used.
Signed-off-by: Christoph Lameter <[email protected]>
---
mm/vmscan.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)
Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c 2007-08-23 13:28:32.000000000 -0700
+++ linux-2.6/mm/vmscan.c 2007-08-23 13:32:42.000000000 -0700
@@ -1106,7 +1106,8 @@ static unsigned long shrink_zone(int pri
}
}
- throttle_vm_writeout(sc->gfp_mask);
+ if (!(sc->gfp_mask & __GFP_NOMEMALLOC))
+ throttle_vm_writeout(sc->gfp_mask);
atomic_dec(&zone->reclaim_in_progress);
return nr_reclaimed;
@@ -1168,6 +1169,9 @@ static unsigned long shrink_zones(int pr
* hope that some of these pages can be written. But if the allocating task
* holds filesystem locks which prevent writeout this might not work, and the
* allocation attempt will fail.
+ *
+ * The __GFP_NOMEMALLOC flag has a special role. If it is set then no memory
+ * allocations or writeout will occur.
*/
unsigned long try_to_free_pages(struct zone **zones, int order, gfp_t gfp_mask)
{
@@ -1180,15 +1184,21 @@ unsigned long try_to_free_pages(struct z
int i;
struct scan_control sc = {
.gfp_mask = gfp_mask,
- .may_writepage = !laptop_mode,
.swap_cluster_max = SWAP_CLUSTER_MAX,
- .may_swap = 1,
.swappiness = vm_swappiness,
.order = order,
};
count_vm_event(ALLOCSTALL);
+ if (gfp_mask & __GFP_NOMEMALLOC) {
+ if (printk_ratelimited())
+ printk(KERN_WARNING "Entering recursive reclaim due "
+ "to depleted memory reserves\n");
+ } else {
+ sc.may_writepage = !laptop_mode;
+ sc.may_swap = 1;
+ }
for (i = 0; zones[i] != NULL; i++) {
struct zone *zone = zones[i];
@@ -1215,6 +1225,9 @@ unsigned long try_to_free_pages(struct z
goto out;
}
+ if (!(gfp_mask & __GFP_NOMEMALLOC))
+ continue;
+
/*
* Try to write back as many pages as we just scanned. This
* tends to cause slow streaming writers to write data to the
Index: linux-2.6/mm/page_alloc.c
===================================================================
--- linux-2.6.orig/mm/page_alloc.c 2007-08-23 13:34:50.000000000 -0700
+++ linux-2.6/mm/page_alloc.c 2007-08-23 13:36:59.000000000 -0700
@@ -1319,6 +1319,20 @@ nofail_alloc:
zonelist, ALLOC_NO_WATERMARKS);
if (page)
goto got_pg;
+ /*
+ * No memory is available at all.
+ *
+ * However, if we are already in reclaim then the
+ * reclaim_state etc is already setup. Simply call
+ * try_to_get_free_pages() with PF_MEMALLOC which
+ * will reclaim without the need to allocate more
+ * memory.
+ */
+ if (p->flags & PF_MEMALLOC && wait &&
+ try_to_free_pages(zonelist->zones, order,
+ gfp_mask | __GFP_NOMEMALLOC))
+ goto restart;
+
if (gfp_mask & __GFP_NOFAIL) {
congestion_wait(WRITE, HZ/50);
goto nofail_alloc;
-
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