Nick has observed that shmem.c still uses GFP_ATOMIC when adding to page
cache or swap cache, without any radix tree preload: so tending to deplete
emergency reserves of memory.
GFP_ATOMIC remains appropriate in shmem_writepage's add_to_swap_cache:
it's being called under memory pressure, so must not wait for more memory
to become available. But shmem_unuse_inode now has a window in which it
can and should preload with GFP_KERNEL, and say GFP_NOWAIT instead of
GFP_ATOMIC in its add_to_page_cache.
shmem_getpage is not so straightforward: its filepage/swappage integrity
relies upon exchanging between caches under spinlock, and it would need a
lot of restructuring to place the preloads correctly. Instead, follow
its pattern of retrying on races: use GFP_NOWAIT instead of GFP_ATOMIC in
add_to_page_cache, and begin each circuit of the repeat loop with a sleeping
radix_tree_preload, followed immediately by radix_tree_preload_end - that
won't guarantee success in the next add_to_page_cache, but doesn't need to.
And we can then remove that bothersome congestion_wait: when needed,
it'll automatically get done in the course of the radix_tree_preload.
Signed-off-by: Hugh Dickins <[email protected]>
---
mm/shmem.c | 25 ++++++++++++++++++-------
1 file changed, 18 insertions(+), 7 deletions(-)
--- tmpfs7/mm/shmem.c 2007-12-06 18:11:26.000000000 +0000
+++ tmpfs8/mm/shmem.c 2007-12-06 19:20:29.000000000 +0000
@@ -901,12 +901,16 @@ found:
error = 1;
if (!inode)
goto out;
+ error = radix_tree_preload(GFP_KERNEL);
+ if (error)
+ goto out;
+ error = 1;
spin_lock(&info->lock);
ptr = shmem_swp_entry(info, idx, NULL);
if (ptr && ptr->val == entry.val)
error = add_to_page_cache(page, inode->i_mapping,
- idx, GFP_ATOMIC);
+ idx, GFP_NOWAIT);
if (error == -EEXIST) {
struct page *filepage = find_get_page(inode->i_mapping, idx);
error = 1;
@@ -931,6 +935,7 @@ found:
if (ptr)
shmem_swp_unmap(ptr);
spin_unlock(&info->lock);
+ radix_tree_preload_end();
out:
unlock_page(page);
page_cache_release(page);
@@ -1185,6 +1190,16 @@ repeat:
goto done;
error = 0;
gfp = mapping_gfp_mask(mapping);
+ if (!filepage) {
+ /*
+ * Try to preload while we can wait, to not make a habit of
+ * draining atomic reserves; but don't latch on to this cpu.
+ */
+ error = radix_tree_preload(gfp & ~__GFP_HIGHMEM);
+ if (error)
+ goto failed;
+ radix_tree_preload_end();
+ }
spin_lock(&info->lock);
shmem_recalc_inode(inode);
@@ -1266,7 +1281,7 @@ repeat:
set_page_dirty(filepage);
swap_free(swap);
} else if (!(error = add_to_page_cache(
- swappage, mapping, idx, GFP_ATOMIC))) {
+ swappage, mapping, idx, GFP_NOWAIT))) {
info->flags |= SHMEM_PAGEIN;
shmem_swp_set(info, entry, 0);
shmem_swp_unmap(entry);
@@ -1280,10 +1295,6 @@ repeat:
spin_unlock(&info->lock);
unlock_page(swappage);
page_cache_release(swappage);
- if (error == -ENOMEM) {
- /* let kswapd refresh zone for GFP_ATOMICs */
- congestion_wait(WRITE, HZ/50);
- }
goto repeat;
}
} else if (sgp == SGP_READ && !filepage) {
@@ -1338,7 +1349,7 @@ repeat:
shmem_swp_unmap(entry);
}
if (error || swap.val || 0 != add_to_page_cache_lru(
- filepage, mapping, idx, GFP_ATOMIC)) {
+ filepage, mapping, idx, GFP_NOWAIT)) {
spin_unlock(&info->lock);
page_cache_release(filepage);
shmem_unacct_blocks(info->flags, 1);
--
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