On Tue, Mar 24, 2015 at 02:37:45PM +0200, Boaz Harrosh wrote:
> On 03/23/2015 02:54 PM, Boaz Harrosh wrote:
> > From: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
> >
> > When freezing an FS, we must write protect all IS_DAX()
> > inodes that have an mmap mapping on an inode. Otherwise
> > application will be able to modify previously faulted-in
> > file pages.
> >
> > I'm actually doing a full unmap_mapping_range because
> > there is no readily available "mapping_write_protect" like
> > functionality. I do not think it is worth it to define one
> > just for here and just for some extra read-faults after an
> > fs_freeze.
> >
> > How hot-path is fs_freeze at all?
> >
>
> OK So reinspecting this was a complete raw RFC. I need to do
> more work on this thing
>
> comments below ...
>
> > CC: Jan Kara <jack@xxxxxxx>
> > CC: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
> > CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
> > ---
> > fs/dax.c | 30 ++++++++++++++++++++++++++++++
> > fs/super.c | 3 +++
> > include/linux/fs.h | 1 +
> > 3 files changed, 34 insertions(+)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index d0bd1f4..f3fc28b 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -549,3 +549,33 @@ int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
> > return dax_zero_page_range(inode, from, length, get_block);
> > }
> > EXPORT_SYMBOL_GPL(dax_truncate_page);
> > +
> > +/* This is meant to be called as part of freeze_super. otherwise we might
> > + * Need some extra locking before calling here.
> > + */
> > +void dax_prepare_freeze(struct super_block *sb)
> > +{
> > + struct inode *inode;
> > +
> > + /* TODO: each DAX fs has some private mount option to enable DAX. If
> > + * We made that option a generic MS_DAX_ENABLE super_block flag we could
> > + * Avoid the 95% extra unneeded loop-on-all-inodes every freeze.
> > + * if (!(sb->s_flags & MS_DAX_ENABLE))
> > + * return 0;
> > + */
> > +
> > + list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
missing locking.
> > + /* TODO: For freezing we can actually do with write-protecting
> > + * the page. But I cannot find a ready made function that does
> > + * that for a giving mapping (with all the proper locking).
> > + * How performance sensitive is the all sb_freeze API?
> > + * For now we can just unmap the all mapping, and pay extra
> > + * on read faults.
> > + */
> > + /* NOTE: Do not unmap private COW mapped pages it will not
> > + * modify the FS.
> > + */
> > + if (IS_DAX(inode))
> > + unmap_mapping_range(inode->i_mapping, 0, 0, 0);
>
> So what happens here is that we loop on all sb->s_inodes every freeze
> and in the not DAX case just do nothing.
Which is real bad and known to be a performance issue. See Josef's
recent sync scalability patchset posting that only tracks and walks
dirty inodes...
> It could be nice to have a flag at the sb level to tel us if we need
> to expect IS_DAX() inodes at all, for example when we are mounted on
> an harddisk it should not be set.
>
> All of ext2/4 and now Dave's xfs have their own
> XFS_MOUNT_DAX / EXT2_MOUNT_DAX / EXT4_MOUNT_DAX
>
> Is it OK if I unify all this on sb->s_flags |= MS_MOUNT_DAX so I can check it
> here in Generic code? The option parsing will be done by each FS but
> the flag be global?
No, because as I mentioned in another thread we're going to end up
with filesystems that don't have "mount wide" DAX behaviour, and we
have to check every dirty inode anyway. And....
> > diff --git a/fs/super.c b/fs/super.c
> > index 2b7dc90..9ef490c 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -1329,6 +1329,9 @@ int freeze_super(struct super_block *sb)
> > /* All writers are done so after syncing there won't be dirty data */
> > sync_filesystem(sb);
> >
> > + /* Need to take care of DAX mmaped inodes */
> > + dax_prepare_freeze(sb);
> > +
>
> So if CONFIG_FS_DAX is not set this will not compile I need to
> define an empty one if not set
... it's the wrong approach - sync_filesystem(sb) shoul dbe handling
this problem, so that sync and fsync work correctly, and then you
don't care about whether DAX is supported or not...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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