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SubjectRe: [PATCH] swap 13/13 may_enter_fs?
On Tue, 25 Mar 2003, Andrew Morton wrote:
>
> For example, a memory-backed filesystem may be trying to allocate GFP_NOFS
> memory while holding filesystem locks which are taken by its writepage.
>
> How about adding a new field to backing_dev_info for this case? Damned if I
> can think of a name for it though.

I think you're overcomplicating it. Of the existing memory_backed bdis
(ramdisk, hugetlbfs, ramfs, sysfs, tmpfs, swap) only two have non-NULL
writepage, and both of those two go (indirectly and directly) to swap
(and neither holds FS lock while waiting to allocate memory).

If we were looking for a correct solution, I don't think backing_dev_info
would be the right place: we're talking about GFP_ needed for writepage,
which should be specified in the struct address_space filled in by the
FS: I think it's more a limitation of the FS than its backing device.

But I don't really want to add another obscure gfp_mask initialization
all over the place. So how about this? As I say, "swap_backed" is
(for the foreseeable future) unnecessary, but does help to correct
mistaken impressions arising from the term "memory_backed".

I've conceded to your delicate sensibility by using if-else rather
than ?: (I used it rather too compactly there, but see nothing wrong
with the construct in general); but you'll be disgusted at my defiance
in not adding a comment in the place you indicated - what would I say?

Hugh

--- 2.5.66-mm1/mm/shmem.c Wed Mar 26 11:51:01 2003
+++ swap13/mm/shmem.c Wed Mar 26 16:35:08 2003
@@ -123,7 +123,8 @@

static struct backing_dev_info shmem_backing_dev_info = {
.ra_pages = 0, /* No readahead */
- .memory_backed = 1, /* Does not contribute to dirty memory */
+ .memory_backed = 1, /* Do not count its dirty pages in nr_dirty */
+ .swap_backed = 1, /* Its memory_backed writepage goes to swap */
};

LIST_HEAD (shmem_inodes);
--- 2.5.66-mm1/mm/swap_state.c Wed Mar 26 11:51:01 2003
+++ swap13/mm/swap_state.c Wed Mar 26 16:35:08 2003
@@ -27,7 +27,8 @@

static struct backing_dev_info swap_backing_dev_info = {
.ra_pages = 0, /* No readahead */
- .memory_backed = 1, /* Does not contribute to dirty memory */
+ .memory_backed = 1, /* Do not count its dirty pages in nr_dirty */
+ .swap_backed = 1, /* Its memory_backed writepage goes to swap */
};

extern struct address_space_operations swap_aops;
--- 2.5.66-mm1/mm/vmscan.c Wed Mar 26 11:51:01 2003
+++ swap13/mm/vmscan.c Wed Mar 26 16:35:08 2003
@@ -235,7 +235,6 @@
pagevec_init(&freed_pvec, 1);
while (!list_empty(page_list)) {
struct page *page;
- int may_enter_fs;

page = list_entry(page_list->prev, struct page, lru);
list_del(&page->lru);
@@ -248,8 +247,6 @@
(*nr_mapped)++;

BUG_ON(PageActive(page));
- may_enter_fs = (gfp_mask & __GFP_FS) ||
- (PageSwapCache(page) && (gfp_mask & __GFP_IO));

if (PageWriteback(page))
goto keep_locked;
@@ -315,15 +312,23 @@
* See swapfile.c:page_queue_congested().
*/
if (PageDirty(page)) {
+ struct backing_dev_info *bdi;
+ unsigned int gfp_needed_for_writepage;
+
if (!is_page_cache_freeable(page))
goto keep_locked;
if (!mapping)
goto keep_locked;
if (mapping->a_ops->writepage == NULL)
goto activate_locked;
- if (!may_enter_fs)
+ bdi = mapping->backing_dev_info;
+ if (bdi->swap_backed)
+ gfp_needed_for_writepage = __GFP_IO;
+ else
+ gfp_needed_for_writepage = __GFP_FS;
+ if (!(gfp_mask & gfp_needed_for_writepage))
goto keep_locked;
- if (!may_write_to_queue(mapping->backing_dev_info))
+ if (!may_write_to_queue(bdi))
goto keep_locked;
write_lock(&mapping->page_lock);
if (test_clear_page_dirty(page)) {
--- 2.5.66-mm1/include/linux/backing-dev.h Wed Mar 26 11:50:36 2003
+++ swap13/include/linux/backing-dev.h Wed Mar 26 16:35:08 2003
@@ -23,7 +23,9 @@
struct backing_dev_info {
unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
unsigned long state; /* Always use atomic bitops on this */
- int memory_backed; /* Cannot clean pages with writepage */
+ unsigned int
+ memory_backed:1,/* Do not count its dirty pages in nr_dirty */
+ swap_backed:1; /* Its memory_backed writepage goes to swap */
};

extern struct backing_dev_info default_backing_dev_info;
-
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