Huge,
I took your advise regarding ~(__GFP_FS|__GFP_IO), AOP_WRITEPAGE_ACTIVATE,
and such. I revised my unionfs_writepage and unionfs_sync_page, and tested
it under memory pressure: I have a couple of live CDs that use tmpfs and can
deterministically reproduce the conditions resulting in A_W_A. I also went
back to using grab_cache_page but with the gfp_mask suggestions you made.
I'm happy to report that it all works great now! Below is the entirety of
the new unionfs_mmap and unionfs_sync_page code. I'd appreciate if you and
others can look it over and see if you find any problems.
Thanks,
Erez.
static int unionfs_writepage(struct page *page, struct writeback_control *wbc)
{
int err = -EIO;
struct inode *inode;
struct inode *lower_inode;
struct page *lower_page;
char *kaddr, *lower_kaddr;
struct address_space *mapping; /* lower inode mapping */
gfp_t old_gfp_mask;
inode = page->mapping->host;
lower_inode = unionfs_lower_inode(inode);
mapping = lower_inode->i_mapping;
/*
* find lower page (returns a locked page)
*
* We turn off __GFP_IO|__GFP_FS so as to prevent a deadlock under
* memory pressure conditions. This is similar to how the loop
* driver behaves (see loop_set_fd in drivers/block/loop.c).
* If we can't find the lower page, we redirty our page and return
* "success" so that the VM will call us again in the (hopefully
* near) future.
*/
old_gfp_mask = mapping_gfp_mask(mapping);
mapping_set_gfp_mask(mapping, old_gfp_mask & ~(__GFP_IO|__GFP_FS));
lower_page = grab_cache_page(mapping, page->index);
mapping_set_gfp_mask(mapping, old_gfp_mask);
if (!lower_page) {
err = 0;
set_page_dirty(page);
goto out;
}
/* get page address, and encode it */
kaddr = kmap(page);
lower_kaddr = kmap(lower_page);
memcpy(lower_kaddr, kaddr, PAGE_CACHE_SIZE);
kunmap(page);
kunmap(lower_page);
BUG_ON(!mapping->a_ops->writepage);
/* call lower writepage (expects locked page) */
clear_page_dirty_for_io(lower_page); /* emulate VFS behavior */
err = mapping->a_ops->writepage(lower_page, wbc);
/* b/c grab_cache_page locked it and ->writepage unlocks on success */
if (err)
unlock_page(lower_page);
/* b/c grab_cache_page increased refcnt */
page_cache_release(lower_page);
if (err < 0) {
ClearPageUptodate(page);
goto out;
}
/*
* Lower file systems such as ramfs and tmpfs, may return
* AOP_WRITEPAGE_ACTIVATE so that the VM won't try to (pointlessly)
* write the page again for a while. But those lower file systems
* also set the page dirty bit back again. Since we successfully
* copied our page data to the lower page, then the VM will come
* back to the lower page (directly) and try to flush it. So we can
* save the VM the hassle of coming back to our page and trying to
* flush too. Therefore, we don't re-dirty our own page, and we
* don't return AOP_WRITEPAGE_ACTIVATE back to the VM (we consider
* this a success).
*/
if (err == AOP_WRITEPAGE_ACTIVATE)
err = 0;
/* all is well */
SetPageUptodate(page);
/* lower mtimes has changed: update ours */
unionfs_copy_attr_times(inode);
unlock_page(page);
out:
return err;
}
static void unionfs_sync_page(struct page *page)
{
struct inode *inode;
struct inode *lower_inode;
struct page *lower_page;
struct address_space *mapping; /* lower inode mapping */
gfp_t old_gfp_mask;
inode = page->mapping->host;
lower_inode = unionfs_lower_inode(inode);
mapping = lower_inode->i_mapping;
/*
* Find lower page (returns a locked page).
*
* We turn off __GFP_IO|__GFP_FS so as to prevent a deadlock under
* memory pressure conditions. This is similar to how the loop
* driver behaves (see loop_set_fd in drivers/block/loop.c).
* If we can't find the lower page, we redirty our page and return
* "success" so that the VM will call us again in the (hopefully
* near) future.
*/
old_gfp_mask = mapping_gfp_mask(mapping);
mapping_set_gfp_mask(mapping, old_gfp_mask & ~(__GFP_IO|__GFP_FS));
lower_page = grab_cache_page(mapping, page->index);
mapping_set_gfp_mask(mapping, old_gfp_mask);
if (!lower_page) {
printk(KERN_ERR "unionfs: grab_cache_page failed\n");
goto out;
}
/* do the actual sync */
/*
* XXX: can we optimize ala RAIF and set the lower page to be
* discarded after a successful sync_page?
*/
if (mapping && mapping->a_ops && mapping->a_ops->sync_page)
mapping->a_ops->sync_page(lower_page);
/* b/c grab_cache_page locked it */
unlock_page(lower_page);
/* b/c grab_cache_page increased refcnt */
page_cache_release(lower_page);
out:
return;
}
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