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Date: Wed, 21 May 2008 01:53:03 +0200
From: Jan Kara <jack@...e.cz>
To: Mingming Cao <cmm@...ibm.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, jack@...e.cz,
pbadari@...ibm.com, linux-ext4@...r.kernel.org,
linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH-v2] JBD: Fix race between free buffer and commit trasanction
> JBD: fix race between journal_try_to_free_buffers() and jbd commit transaction
>
> From: Mingming Cao <cmm@...ibm.com>
>
> journal_try_to_free_buffers() could race with jbd commit transaction when
> the later is holding the buffer reference while waiting for the data buffer
> to flush to disk. If the caller of journal_try_to_free_buffers() request
> tries hard to release the buffers, it will treat the failure as error and return
> back to the caller. We have seen the directo IO failed due to this race.
> Some of the caller of releasepage() also expecting the buffer to be dropped
> when passed with GFP_KERNEL mask to the releasepage()->journal_try_to_free_buffers().
>
> With this patch, if the caller is passing the GFP_KERNEL to indicating this
> call could wait, in case of try_to_free_buffers() failed, let's waiting for
> journal_commit_transaction() to finish commit the current committing transaction
> , then try to free those buffers again with journal locked.
>
> Signed-off-by: Mingming Cao <cmm@...ibm.com>
> Reviewed-by: Badari Pulavarty <pbadari@...ibm.com>
> ---
> fs/jbd/transaction.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++--
> mm/filemap.c | 3 --
> 2 files changed, 54 insertions(+), 4 deletions(-)
>
> Index: linux-2.6.26-rc2/fs/jbd/transaction.c
> ===================================================================
> --- linux-2.6.26-rc2.orig/fs/jbd/transaction.c 2008-05-11 17:09:41.000000000 -0700
> +++ linux-2.6.26-rc2/fs/jbd/transaction.c 2008-05-19 16:16:41.000000000 -0700
> @@ -1648,12 +1648,39 @@ out:
> return;
> }
>
> +/*
> + * journal_try_to_free_buffers() could race with journal_commit_transaction()
> + * The later might still hold the reference count to the buffers when inspecting
> + * them on t_syncdata_list or t_locked_list.
> + *
> + * Journal_try_to_free_buffers() will call this function to
> + * wait for the current transaction to finish syncing data buffers, before
> + * try to free that buffer.
> + *
> + * Called with journal->j_state_lock hold.
> + */
> +static void journal_wait_for_transaction_sync_data(journal_t *journal)
> +{
> + transaction_t *transaction = NULL;
> + tid_t tid;
> +
> + transaction = journal->j_committing_transaction;
> +
> + if (!transaction)
> + return;
> +
> + tid = transaction->t_tid;
> + spin_unlock(&journal->j_state_lock);
> + log_wait_commit(journal, tid);
> + spin_lock(&journal->j_state_lock);
> +}
What is actually the point of entering the function with j_state_lock
held and also keeping it after return? It should be enough to take it
and release it just inside this function, shouldn't it?
> /**
> * int journal_try_to_free_buffers() - try to free page buffers.
> * @journal: journal for operation
> * @page: to try and free
> - * @unused_gfp_mask: unused
> + * @gfp_mask: specifies whether the call may block
> + * (__GFP_WAIT & __GFP_FS via GFP_KERNEL)
This comment seems a bit misleading to me - I'd rather write there:
@gfp_mask: we use the mask to detect how hard should we try to release
buffers. If __GFP_WAIT and __GFP_FS is set, we wait for commit code to
release the buffers.
> *
> *
> * For all the buffers on this page,
> @@ -1682,9 +1709,11 @@ out:
> * journal_try_to_free_buffer() is changing its state. But that
> * cannot happen because we never reallocate freed data as metadata
> * while the data is part of a transaction. Yes?
> + *
> + * Return 0 on failure, 1 on success
> */
> int journal_try_to_free_buffers(journal_t *journal,
> - struct page *page, gfp_t unused_gfp_mask)
> + struct page *page, gfp_t gfp_mask)
> {
> struct buffer_head *head;
> struct buffer_head *bh;
> @@ -1713,7 +1742,30 @@ int journal_try_to_free_buffers(journal_
> if (buffer_jbd(bh))
> goto busy;
> } while ((bh = bh->b_this_page) != head);
> +
> ret = try_to_free_buffers(page);
> +
> + /*
> + * There are a number of places where journal_try_to_free_buffers()
> + * could race with journal_commit_transaction(), the later still
> + * holds the reference to the buffers to free while processing them.
> + * try_to_free_buffers() failed to free those buffers. Some of the
> + * caller of releasepage() request page buffers to be dropped, otherwise
> + * treat the fail-to-free as errors (such as generic_file_direct_IO())
> + *
> + * So, if the caller of try_to_release_page() wants the synchronous
> + * behaviour(i.e make sure buffers are dropped upon return),
> + * let's wait for the current transaction to finish flush of
> + * dirty data buffers, then try to free those buffers again,
> + * with the journal locked.
> + */
> + if (ret == 0 && gfp_mask & GFP_KERNEL) {
I think this test is wrong - it should rather be something like
(ret == 0 && (gfp_mask & GFP_KERNEL == GFP_KERNEL)) - or even expand the
test to gfp_mask & __GFP_WAIT && gfp_mask & __GFP_FS && gfp_mask &
__GFP_IO.
> + spin_lock(&journal->j_state_lock);
> + journal_wait_for_transaction_sync_data(journal);
> + ret = try_to_free_buffers(page);
> + spin_unlock(&journal->j_state_lock);
> + }
> +
> busy:
> return ret;
> }
> Index: linux-2.6.26-rc2/mm/filemap.c
> ===================================================================
> --- linux-2.6.26-rc2.orig/mm/filemap.c 2008-05-19 16:00:01.000000000 -0700
> +++ linux-2.6.26-rc2/mm/filemap.c 2008-05-19 16:01:34.000000000 -0700
> @@ -2581,9 +2581,8 @@ out:
> * Otherwise return zero.
> *
> * The @gfp_mask argument specifies whether I/O may be performed to release
> - * this page (__GFP_IO), and whether the call may block (__GFP_WAIT).
> + * this page (__GFP_IO), and whether the call may block (__GFP_WAIT & __GFP_FS).
Probably __GFP_WAIT | __GFP_IO here... But I'm not sure why do we
really change this...
> *
> - * NOTE: @gfp_mask may go away, and this function may become non-blocking.
> */
> int try_to_release_page(struct page *page, gfp_t gfp_mask)
> {
>
Honza
--
Jan Kara <jack@...e.cz>
SuSE CR Labs
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