Re: [PATCH v2 07/18] btrfs: extent_io: make grab_extent_buffer_from_page() to handle subpage case

[Qu Wenruo]

On 2020/12/10 下午11:39, Nikolay Borisov wrote:

On 10.12.20 г. 8:38 ч., Qu Wenruo wrote:

For subpage case, grab_extent_buffer_from_page() can't really get an
extent buffer just from btrfs_subpage.

Although we have btrfs_subpage::tree_block_bitmap, which can be used to
grab the bytenr of an existing extent buffer, and can then go radix tree
search to grab that existing eb.

However we are still doing radix tree insert check in
alloc_extent_buffer(), thus we don't really need to do the extra hassle,
just let alloc_extent_buffer() to handle existing eb in radix tree.

So for grab_extent_buffer_from_page(), just always return NULL for
subpage case.

Signed-off-by: Qu Wenruo <w...@suse.com>
---
  fs/btrfs/extent_io.c | 13 +++++++++++--
  1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 51dd7ec3c2b3..b99bd0402130 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5278,10 +5278,19 @@ struct extent_buffer *alloc_test_extent_buffer(struct 
btrfs_fs_info *fs_info,
  }
  #endif

-static struct extent_buffer *grab_extent_buffer_from_page(struct page *page)
+static struct extent_buffer *grab_extent_buffer_from_page(
+               struct btrfs_fs_info *fs_info, struct page *page)
  {
        struct extent_buffer *exists;

+       /*
+        * For subpage case, we completely rely on radix tree to ensure we
+        * don't try to insert two eb for the same bytenr.
+        * So here we alwasy return NULL and just continue.
+        */
+       if (fs_info->sectorsize < PAGE_SIZE)
+               return NULL;
+

Instead of hiding this in the function, just open-code it in the only caller. 
It would look like:

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index b99bd0402130..440dab207944 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5370,8 +5370,9 @@ struct extent_buffer *alloc_extent_buffer(struct 
btrfs_fs_info *fs_info,
                 }

                 spin_lock(&mapping->private_lock);
-               exists = grab_extent_buffer_from_page(fs_info, p);
-               if (exists) {
+               if (fs_info->sectorsize == PAGE_SIZE &&
+                   (exists = grab_extent_buffer_from_page(fs_info, p)));
+               {
                         spin_unlock(&mapping->private_lock);
                         unlock_page(p);
                         put_page(p);


Admittedly that exist = ... in the if condition is a bit of an anti-pattern but 
given it's used in only 1 place
and makes the flow of code more linear I'd say it's a win. But would like to 
hear David's opinion.

Personally speaking, the (exists == *) inside the condition really looks
ugly and hard to grasp.

And since grab_extent_buffer_from_page() is only called once, the
generated code shouldn't be that much different anyway as the compiler
would mostly just inline it.

So I still prefer the current code, not to mention it also provides
extra space for the comment.

Thanks,
Qu

        /* Page not yet attached to an extent buffer */
        if (!PagePrivate(page))
                return NULL;
@@ -5361,7 +5370,7 @@ struct extent_buffer *alloc_extent_buffer(struct 
btrfs_fs_info *fs_info,
                }

                spin_lock(&mapping->private_lock);
-               exists = grab_extent_buffer_from_page(p);
+               exists = grab_extent_buffer_from_page(fs_info, p);
                if (exists) {
                        spin_unlock(&mapping->private_lock);
                        unlock_page(p);