Re: [BUG] 2.4 VM sucks. Again

Andrew Morton
2002-05-24T19:34:06Z

"Martin J. Bligh" wrote:
>
> >> Sounds like exactly the same problem we were having. There are two
> >> approaches to solving this - Andrea has a patch that tries to free them
> >> under memory pressure, akpm has a patch that hacks them down as soon
> >> as you've fininshed with them (posted to lse-tech mailing list). Both
> >> approaches seemed to work for me, but the performance of the fixes still
> >> has to be established.
> >
> > Where can I find the akpm patch?
>
> http://marc.theaimsgroup.com/?l=lse-tech&m=102083525007877&w=2
>
> > Any plans to merge this into the main kernel, giving a choice
> > (in config or /proc) to enable this?
>
> I don't think Andrew is ready to submit this yet ... before anything
> gets merged back, it'd be very worthwhile testing the relative
> performance of both solutions ... the more testers we have the
> better ;-)
>

Cripes no. It's pretty experimental. Andrea spotted a bug, too. Fixed
version is below.

It's possible that keeping the number of buffers as low as possible
will give improved performance over Andrea's approach because it
leaves more ZONE_NORMAL for other things. It's also possible that
it'll give worse performance because more get_block's need to be
done for file overwriting.

--- 2.4.19-pre8/include/linux/pagemap.h~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/include/linux/pagemap.h Fri May 24 12:26:30 2002
@@ -89,13 +89,7 @@ extern void add_to_page_cache(struct pag
extern void add_to_page_cache_locked(struct page * page, struct address_space *mapping, unsigned long index);
extern int add_to_page_cache_unique(struct page * page, struct address_space *mapping, unsigned long index, struct page **hash);

-extern void ___wait_on_page(struct page *);
-
-static inline void wait_on_page(struct page * page)
-{
- if (PageLocked(page))
- ___wait_on_page(page);
-}
+extern void wait_on_page(struct page *);

extern struct page * grab_cache_page (struct address_space *, unsigned long);
extern struct page * grab_cache_page_nowait (struct address_space *, unsigned long);
--- 2.4.19-pre8/mm/filemap.c~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/mm/filemap.c Fri May 24 12:24:56 2002
@@ -608,7 +608,7 @@ int filemap_fdatawait(struct address_spa
page_cache_get(page);
spin_unlock(&pagecache_lock);

- ___wait_on_page(page);
+ wait_on_page(page);
if (PageError(page))
ret = -EIO;

@@ -805,33 +805,29 @@ static inline wait_queue_head_t *page_wa
return &wait[hash];
}

-/*
- * Wait for a page to get unlocked.
+static void kill_buffers(struct page *page)
+{
+ if (!PageLocked(page))
+ BUG();
+ if (page->buffers)
+ try_to_release_page(page, GFP_NOIO);
+}
+
+/*
+ * Wait for a page to come unlocked. Then try to ditch its buffer_heads.
*
- * This must be called with the caller "holding" the page,
- * ie with increased "page->count" so that the page won't
- * go away during the wait..
+ * FIXME: Make the ditching dependent on CONFIG_MONSTER_BOX or something.
*/
-void ___wait_on_page(struct page *page)
+void wait_on_page(struct page *page)
{
- wait_queue_head_t *waitqueue = page_waitqueue(page);
- struct task_struct *tsk = current;
- DECLARE_WAITQUEUE(wait, tsk);
-
- add_wait_queue(waitqueue, &wait);
- do {
- set_task_state(tsk, TASK_UNINTERRUPTIBLE);
- if (!PageLocked(page))
- break;
- sync_page(page);
- schedule();
- } while (PageLocked(page));
- __set_task_state(tsk, TASK_RUNNING);
- remove_wait_queue(waitqueue, &wait);
+ lock_page(page);
+ kill_buffers(page);
+ unlock_page(page);
}
+EXPORT_SYMBOL(wait_on_page);

/*
- * Unlock the page and wake up sleepers in ___wait_on_page.
+ * Unlock the page and wake up sleepers in lock_page.
*/
void unlock_page(struct page *page)
{
@@ -1400,6 +1396,11 @@ found_page:

if (!Page_Uptodate(page))
goto page_not_up_to_date;
+ if (page->buffers) {
+ lock_page(page);
+ kill_buffers(page);
+ unlock_page(page);
+ }
generic_file_readahead(reada_ok, filp, inode, page);
page_ok:
/* If users can be writing to this page using arbitrary
@@ -1457,6 +1458,7 @@ page_not_up_to_date:

/* Did somebody else fill it already? */
if (Page_Uptodate(page)) {
+ kill_buffers(page);
UnlockPage(page);
goto page_ok;
}
@@ -1948,6 +1950,11 @@ retry_find:
*/
if (!Page_Uptodate(page))
goto page_not_uptodate;
+ if (page->buffers) {
+ lock_page(page);
+ kill_buffers(page);
+ unlock_page(page);
+ }

success:
/*
@@ -2006,6 +2013,7 @@ page_not_uptodate:

/* Did somebody else get it up-to-date? */
if (Page_Uptodate(page)) {
+ kill_buffers(page);
UnlockPage(page);
goto success;
}
@@ -2033,6 +2041,7 @@ page_not_uptodate:

/* Somebody else successfully read it in? */
if (Page_Uptodate(page)) {
+ kill_buffers(page);
UnlockPage(page);
goto success;
}
@@ -2850,6 +2859,7 @@ retry:
goto retry;
}
if (Page_Uptodate(page)) {
+ kill_buffers(page);
UnlockPage(page);
goto out;
}
--- 2.4.19-pre8/kernel/ksyms.c~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/kernel/ksyms.c Fri May 24 12:24:56 2002
@@ -202,7 +202,6 @@ EXPORT_SYMBOL(ll_rw_block);
EXPORT_SYMBOL(submit_bh);
EXPORT_SYMBOL(unlock_buffer);
EXPORT_SYMBOL(__wait_on_buffer);
-EXPORT_SYMBOL(___wait_on_page);
EXPORT_SYMBOL(generic_direct_IO);
EXPORT_SYMBOL(discard_bh_page);
EXPORT_SYMBOL(block_write_full_page);
--- 2.4.19-pre8/mm/vmscan.c~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/mm/vmscan.c Fri May 24 12:24:56 2002
@@ -365,8 +365,13 @@ static int shrink_cache(int nr_pages, zo
if (unlikely(!page_count(page)))
continue;

- if (!memclass(page_zone(page), classzone))
+ if (!memclass(page_zone(page), classzone)) {
+ if (page->buffers && !TryLockPage(page)) {
+ try_to_release_page(page, GFP_NOIO);
+ unlock_page(page);
+ }
continue;
+ }

/* Racy check to avoid trylocking when not worthwhile */
if (!page->buffers && (page_count(page) != 1 || !page->mapping))
@@ -562,6 +567,11 @@ static int shrink_caches(zone_t * classz
nr_pages -= kmem_cache_reap(gfp_mask);
if (nr_pages <= 0)
return 0;
+ if ((gfp_mask & __GFP_WAIT) && (shrink_buffer_cache() > 16)) {
+ nr_pages -= kmem_cache_reap(gfp_mask);
+ if (nr_pages <= 0)
+ return 0;
+ }

nr_pages = chunk_size;
/* try to keep the active list 2/3 of the size of the cache */
--- 2.4.19-pre8/fs/buffer.c~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/fs/buffer.c Fri May 24 12:26:28 2002
@@ -1500,6 +1500,10 @@ static int __block_write_full_page(struc
/* Stage 3: submit the IO */
do {
struct buffer_head *next = bh->b_this_page;
+ /*
+ * Stick it on BUF_LOCKED so shrink_buffer_cache() can nail it.
+ */
+ refile_buffer(bh);
submit_bh(WRITE, bh);
bh = next;
} while (bh != head);
@@ -2615,6 +2619,25 @@ static int sync_page_buffers(struct buff
int try_to_free_buffers(struct page * page, unsigned int gfp_mask)
{
struct buffer_head * tmp, * bh = page->buffers;
+ int was_uptodate = 1;
+
+ if (!PageLocked(page))
+ BUG();
+
+ if (!bh)
+ return 1;
+ /*
+ * Quick check for freeable buffers before we go take three
+ * global locks.
+ */
+ if (!(gfp_mask & __GFP_IO)) {
+ tmp = bh;
+ do {
+ if (buffer_busy(tmp))
+ return 0;
+ tmp = tmp->b_this_page;
+ } while (tmp != bh);
+ }

cleaned_buffers_try_again:
spin_lock(&lru_list_lock);
@@ -2637,7 +2660,8 @@ cleaned_buffers_try_again:
tmp = tmp->b_this_page;

if (p->b_dev == B_FREE) BUG();
-
+ if (!buffer_uptodate(p))
+ was_uptodate = 0;
remove_inode_queue(p);
__remove_from_queues(p);
__put_unused_buffer_head(p);
@@ -2645,7 +2669,15 @@ cleaned_buffers_try_again:
spin_unlock(&unused_list_lock);

/* Wake up anyone waiting for buffer heads */
- wake_up(&buffer_wait);
+ smp_mb();
+ if (waitqueue_active(&buffer_wait))
+ wake_up(&buffer_wait);
+
+ /*
+ * Make sure we don't read buffers again when they are reattached
+ */
+ if (was_uptodate)
+ SetPageUptodate(page);

/* And free the page */
page->buffers = NULL;
@@ -2674,6 +2706,62 @@ busy_buffer_page:
}
EXPORT_SYMBOL(try_to_free_buffers);

+/*
+ * Returns the number of pages which might have become freeable
+ */
+int shrink_buffer_cache(void)
+{
+ struct buffer_head *bh;
+ int nr_todo;
+ int nr_shrunk = 0;
+
+ /*
+ * Move any clean unlocked buffers from BUF_LOCKED onto BUF_CLEAN
+ */
+ spin_lock(&lru_list_lock);
+ for ( ; ; ) {
+ bh = lru_list[BUF_LOCKED];
+ if (!bh || buffer_locked(bh))
+ break;
+ __refile_buffer(bh);
+ }
+
+ /*
+ * Now start liberating buffers
+ */
+ nr_todo = nr_buffers_type[BUF_CLEAN];
+ while (nr_todo--) {
+ struct page *page;
+
+ bh = lru_list[BUF_CLEAN];
+ if (!bh)
+ break;
+
+ /*
+ * Park the buffer on BUF_LOCKED so we don't revisit it on
+ * this pass.
+ */
+ __remove_from_lru_list(bh);
+ bh->b_list = BUF_LOCKED;
+ __insert_into_lru_list(bh, BUF_LOCKED);
+ page = bh->b_page;
+ if (TryLockPage(page))
+ continue;
+
+ page_cache_get(page);
+ spin_unlock(&lru_list_lock);
+ if (try_to_release_page(page, GFP_NOIO))
+ nr_shrunk++;
+ unlock_page(page);
+ page_cache_release(page);
+ spin_lock(&lru_list_lock);
+ }
+ spin_unlock(&lru_list_lock);
+// printk("%s: liberated %d page's worth of buffer_heads\n",
+// __FUNCTION__, nr_shrunk);
+ return (nr_shrunk * sizeof(struct buffer_head)) / PAGE_CACHE_SIZE;
+}
+
/* ================== Debugging =================== */

void show_buffers(void)
@@ -2988,6 +3076,7 @@ int kupdate(void *startup)
#ifdef DEBUG
printk(KERN_DEBUG "kupdate() activated...\n");
#endif
+ shrink_buffer_cache();
sync_old_buffers();
run_task_queue(&tq_disk);
}
--- 2.4.19-pre8/include/linux/fs.h~nuke-buffers Fri May 24 12:24:56 2002
+++ 2.4.19-pre8-akpm/include/linux/fs.h Fri May 24 12:24:56 2002
@@ -1116,6 +1116,7 @@ extern int FASTCALL(try_to_free_buffers(
extern void refile_buffer(struct buffer_head * buf);
extern void create_empty_buffers(struct page *, kdev_t, unsigned long);
extern void end_buffer_io_sync(struct buffer_head *bh, int uptodate);
+extern int shrink_buffer_cache(void);

/* reiserfs_writepage needs this */
extern void set_buffer_async_io(struct buffer_head *bh) ;

-