Hi.
On Sat, 23 Apr 2011 00:10:25 +0200
Jan Kara <jack@xxxxxxx> wrote:
> On Fri 22-04-11 15:58:39, Toshiyuki Okajima wrote:
> > I have confirmed that the following patch works fine while my or
> > Mizuma-san's reproducer is running. Therefore,
> > we can block to write the data, which is mmapped to a file, into a disk
> > by a page-fault while fsfreezing.
> >
> > I think this patch fixes the following two problems:
> > - A deadlock occurs between ext4_da_writepages() (called from
> > writeback_inodes_wb) and thaw_super(). (reported by Mizuma-san)
> > - We can also write the data, which is mmapped to a file,
> > into a disk while fsfreezing (ext3/ext4).
> > (reported by me)
> >
> > Please examine this patch.
> Thanks for the patch. The ext3 part is not as easy as this. You cannot
> really get i_alloc_sem in ext3_page_mkwrite() because mmap_sem is already
> held by page fault code and i_alloc_sem should be acquired before it (yes I
> know, ext4 already has this bug which should be fixed when I get to it).
> Also you'll find that performance of random writers via mmap (which is
> relatively common) is going to be rather bad with this patch (because the
> file will be heavily fragmented). We have to be more clever which is
> exactly why it's taking me so long with my patch :) But tests are already
> running so if everything goes fine, I should have patches to submit next
> week.
OK, I'll wait your patch. :)
>
> The ext4 part looks correct. I'd just also like to have some comments about
> how freeze handling is done because it's kind of subtle.
How about this?
Thanks,
Toshiyuki Okajima
----------------------------------------------------------------------------------------------------
Subject: [PATCH] ext4: prevent the mmapped page flushing to disk while fsfreezing
Signed-off-by: Toshiyuki Okajima <toshi.okajima@xxxxxxxxxxxxxx>
---
fs/ext4/inode.c | 10 +++++++++-
1 files changed, 9 insertions(+), 1 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index f2fa5e8..411b177 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5812,7 +5812,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
}
ret = 0;
if (PageMappedToDisk(page))
- goto out_unlock;
+ goto out_frozen;
if (page->index == size >> PAGE_CACHE_SHIFT)
len = size & ~PAGE_CACHE_MASK;
@@ -5830,6 +5830,14 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
ext4_bh_unmapped)) {
unlock_page(page);
+out_frozen:
+ /*
+ * We must wait here while the filesystem is being
+ * frozen otherwise a flushing thread can write this
+ * page to the disk (we can update the filesystem even
+ * if it is frozen).
+ */
+ vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
goto out_unlock;
}
}
--
1.5.5.6
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