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

[Namjae Jeon <namjae.jeon@samsung.com>]

> 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.

Thanks.
> 								Honza
> >
> > ---
> >  fs/jbd2/checkpoint.c | 11 +++++++++++
> >  fs/jbd2/commit.c     |  2 +-
> >  include/linux/jbd2.h |  1 +
> >  3 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
> > index 4227dc4..bf68442 100644
> > --- a/fs/jbd2/checkpoint.c
> > +++ b/fs/jbd2/checkpoint.c
> > @@ -529,6 +529,7 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
> >  	transaction_t *transaction;
> >  	journal_t *journal;
> >  	int ret = 0;
> > +	struct buffer_head *bh;
> >
> >  	JBUFFER_TRACE(jh, "entry");
> >
> > @@ -538,10 +539,20 @@ int __jbd2_journal_remove_checkpoint(struct journal_head *jh)
> >  	}
> >  	journal = transaction->t_journal;
> >
> > +	bh = jh2bh(jh);
> > +	get_bh(bh);
> >  	JBUFFER_TRACE(jh, "removing from transaction");
> >  	__buffer_unlink(jh);
> >  	jh->b_cp_transaction = NULL;
> >  	jbd2_journal_put_journal_head(jh);
> > +	/*
> > +	 * if journal head is freed, try to free buffers from a truncated
> > +	 * page, if page buffers are not busy and page->mapping is NULL
> > +	 */
> > +	if (!buffer_jbd(bh))
> > +		release_buffer_page(bh);
> > +	else
> > +		__brelse(bh);
> >
> >  	if (transaction->t_checkpoint_list != NULL ||
> >  	    transaction->t_checkpoint_io_list != NULL)
> > diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> > index b73e021..d94ec3f 100644
> > --- a/fs/jbd2/commit.c
> > +++ b/fs/jbd2/commit.c
> > @@ -63,7 +63,7 @@ static void journal_end_buffer_io_sync(struct buffer_head *bh, int uptodate)
> >   * Called under lock_journal(), and possibly under journal_datalist_lock.  The
> >   * caller provided us with a ref against the buffer, and we drop that here.
> >   */
> > -static void release_buffer_page(struct buffer_head *bh)
> > +void release_buffer_page(struct buffer_head *bh)
> >  {
> >  	struct page *page;
> >
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index edb640a..523f345 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -1039,6 +1039,7 @@ int __jbd2_update_log_tail(journal_t *journal, tid_t tid, unsigned long block);
> >  void jbd2_update_log_tail(journal_t *journal, tid_t tid, unsigned long block);
> >
> >  /* Commit management */
> > +extern void release_buffer_page(struct buffer_head *);
> >  extern void jbd2_journal_commit_transaction(journal_t *);
> >
> >  /* Checkpoint list management */
> >
> > ----------------------------------------------------------
> > >
> > > 					- Ted
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
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> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR