On Tue, Dec 8, 2020 at 12:20 PM Dave Chinner <david@xxxxxxxxxxxxx> wrote:
>
> On Tue, Dec 08, 2020 at 10:15:43AM +0800, Yafang Shao wrote:
> > PF_FSTRANS which is used to avoid transaction reservation recursion, is
> > dropped since commit 9070733b4efa ("xfs: abstract PF_FSTRANS to
> > PF_MEMALLOC_NOFS") and commit 7dea19f9ee63 ("mm: introduce
> > memalloc_nofs_{save,restore} API") and replaced by PF_MEMALLOC_NOFS which
> > means to avoid filesystem reclaim recursion.
> >
> > As these two flags have different meanings, we'd better reintroduce
> > PF_FSTRANS back. To avoid wasting the space of PF_* flags in task_struct,
> > we can reuse the current->journal_info to do that, per Willy. As the
> > check of transaction reservation recursion is used by XFS only, we can
> > move the check into xfs_vm_writepage(s), per Dave.
> >
> > To better abstract that behavoir, two new helpers are introduced, as
> > follows,
> > - xfs_trans_context_active
> > To check whehter current is in fs transcation or not
> > - xfs_trans_context_swap
> > Transfer the transaction context when rolling a permanent transaction
> >
> > These two new helpers are instroduced in xfs_trans.h.
> >
> > Cc: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: David Howells <dhowells@xxxxxxxxxx>
> > Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> > fs/iomap/buffered-io.c | 7 -------
> > fs/xfs/xfs_aops.c | 17 +++++++++++++++++
> > fs/xfs/xfs_trans.c | 3 +++
> > fs/xfs/xfs_trans.h | 25 +++++++++++++++++++++++++
> > 4 files changed, 45 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 10cc7979ce38..3c53fa6ce64d 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -1458,13 +1458,6 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
> > PF_MEMALLOC))
> > goto redirty;
> >
> > - /*
> > - * Given that we do not allow direct reclaim to call us, we should
> > - * never be called in a recursive filesystem reclaim context.
> > - */
> > - if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
> > - goto redirty;
> > -
> > /*
> > * Is this page beyond the end of the file?
> > *
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 2371187b7615..28db93d0da97 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -568,6 +568,16 @@ xfs_vm_writepage(
> > {
> > struct xfs_writepage_ctx wpc = { };
> >
> > + /*
> > + * Given that we do not allow direct reclaim to call us, we should
> > + * never be called while in a filesystem transaction.
> > + */
> > + if (xfs_trans_context_active()) {
> > + redirty_page_for_writepage(wbc, page);
> > + unlock_page(page);
> > + return 0;
> > + }
>
> hmmm. Missing warning....
>
> > diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> > index 44b11c64a15e..82c6735e40fc 100644
> > --- a/fs/xfs/xfs_trans.h
> > +++ b/fs/xfs/xfs_trans.h
> > @@ -268,16 +268,41 @@ xfs_trans_item_relog(
> > return lip->li_ops->iop_relog(lip, tp);
> > }
> >
> > +static inline bool
> > +xfs_trans_context_active(void)
> > +{
> > + /* Use journal_info to indicate current is in a transaction */
> > + if (WARN_ON_ONCE(current->journal_info != NULL))
> > + return true;
> > +
> > + return false;
> > +}
>
> Ah, this is wrong. The call sites should be:
>
> if (WARN_ON_ONCE(xfs_trans_context_active())) {
> /* do error handling */
> return error_value;
> }
>
> because we might want to use xfs_trans_context_active() to check if
> we are in a transaction context or not and that should not generate
> a warning. Also, placing the warning at the call site gives a more
> accurate indication of which IO path generated the warning....
>
Thanks for the explanation. I will update it in the next version.
--
Thanks
Yafang
