Re: memory leak: data=journal and {collapse,insert,zero}_range

[Jan Kara <jack@suse.cz>]

On Tue 17-11-15 13:47:50, Namjae Jeon wrote:
> > On Mon 09-11-15 14:21:11, Namjae Jeon wrote:
> > > >
> > > > On Wed, Oct 21, 2015 at 06:44:10PM +0900, Namjae Jeon wrote:
> > > > > > Interestingly we're not seeing these memory leaks on the truncate
> > > > > > path, so I suspect the issue is in how collapse range is clearing
> > > > > > pages from the page cache, especially pages that were freshly written
> > > > > > to the journal by the commit but which hadn't yet been writtten to
> > > > > > disk and then marked as complete so we can allow the relevant
> > > > > > transaction to be checkpointed.  (Although we're not leaking the
> > > > > > journal head structures, but only the buffer heads, so the story most
> > > > > > be a bit more complicated than that.)
> > > > >
> > > > > Okay, Thanks for sharing your view and points !!
> > > > >
> > > > > Currently I can reproduce memory leak issue without collase/insert/zero range.
> > > > > conditions like the following.(collase/insert/zero range are disable with -I -C -z option and
> > add -y
> > > > option instead of -W)
> > > > >   1. small size parition(1GB)
> > > > >   2. run fsx with these options "./fsx -N 30000 -o 128000 -l 500000 -r 4096 -t 512 -w 512 -Z -R
> > -y -
> > > > I -C -z testfile"
> > > > > And same result with generic/091 is showing (buffer_head leak)
> > > > >
> > > > > So I am starting to find root-cause base on your points.
> > > > > I will share the result or the patch.
> > > >
> > > > Thanks, that's very interesting data point.  So this makes it appear
> > > > that the problem *is* probably with how we deal with checkpointing
> > > > buffers after the pages get discarded using either a truncate or a
> > > > collapse_range, since the 'y' option causes a lot fsync's, and hence
> > > > commits, some of which are happening after a truncate command.
> > > >
> > > > Thanks for a taking a look at this.  I really appreciate it.
> > > >
> > > > Cheers,
> > >
> > >
> > > Hi Ted,
> > >
> > > Could you review this patch?
> > >
> > > Thanks!
> > > ---------------------------------------------------------------------------------
> > > Subject: [PATCH] jbd2: try to free buffers from truncated page after
> > >  checkpoint
> > >
> > > when ext4 is mounted in data=journal mode, and truncate operation
> > > such as settatr(size), collopse, insert and zero range are used, there are
> > > are many truncated pages with NULL page->mapping. Such truncated pages
> > > pile up quickly due to truncate_pagecache on data pages associated with journal.
> > > As page->mapping is NULL for such truncated pages, they are not freed
> > > by drop cache(3) or umount. As a result, MemFree in /proc/meminfo decreases
> > > quickly and active buffer_head slab objects grow in /proc/slabinfo.
> > > This patch attempts to free buffers from such pages at the end of jbd2
> > > checkpoint, if pages do not have any busy buffers and NULL mapping.
> > 
> > Hum, why such pages didn't get freed by release_buffer_page() call
> > happening when processing transaction's forget list? Because the idea is
> > that such pages should be discarded at that point...
> Hi Jan,
> 
> When I checked this code, release_buffer_page is not called
> when buffer_jbddirty is true. Such buffers with JBD_Dirty are added
> to new checkpoint.
> 
>                 if (buffer_jbddirty(bh)) {
>                        JBUFFER_TRACE(jh, "add to new checkpointing trans");
>                        __jbd2_journal_insert_checkpoint(jh, commit_transaction);
>                        if (is_journal_aborted(journal))
>                                clear_buffer_jbddirty(bh);
>                 } else {
>                        J_ASSERT_BH(bh, !buffer_dirty(bh));
>                        /*
>                         * The buffer on BJ_Forget list and not jbddirty means
>                         * it has been freed by this transaction and hence it
>                         * could not have been reallocated until this
>                         * transaction has committed. *BUT* it could be
>                         * reallocated once we have written all the data to
>                         * disk and before we process the buffer on BJ_Forget
>                         * list.
>                         */
>                        if (!jh->b_next_transaction)
>                                try_to_free = 1;
>                 }
>                 JBUFFER_TRACE(jh, "refile or unfile buffer");
>                 __jbd2_journal_refile_buffer(jh);
>                 jbd_unlock_bh_state(bh);
> 
> Next buffer was unfiled by __jbd2_journal_refile_buffer, JBD_Dirty cleared and
> BH_Dirty was set in same function. Later it must have been written back as
> BH_Dirty was cleared.
> And ext4_wait_for_tail_page_commit-> __ext4_journalled_invalidatepage ->
> journal_unmap_buffer zaps the buffer:
> 
>                 if (!buffer_dirty(bh)) {
>                         /* bdflush has written it.  We can drop it now */
>                         goto zap_buffer;
>                 }
> 
> next, truncate_pagecache is called, which clears the page mapping.
> Eventually, remove checkpoint is called, but such page with NULL mapping was
> not freed. So, I had added release_buffer_page at the end of remove checkpoint
> to attempt to free such free buffer pages. Please let me know your opinion.

OK, thanks for the detailed analysis. But when the buffer gets truncated,
jbd2_journal_invalidatepage() either removes the buffer from the
transaction (obviously didn't happen here) or it sets buffer_freed and
buffer_jbddirty should get cleared when processing the BJ_Forget list. So
why that didn't happen? Can you have a look into what
jbd2_journal_invalidatepage() did to buffer in that page?

Generally truncated buffers should not enter checkpoint list since writing
them is just a waste of disk bandwidth...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR