Re: [PATCH v2 31/34] block: use file->f_op to indicate restricted writes

[Christian Brauner <brauner@xxxxxxxxxx>]

On Thu, Feb 01, 2024 at 12:08:58PM +0100, Jan Kara wrote:
> On Tue 23-01-24 14:26:48, Christian Brauner wrote:
> > Make it possible to detected a block device that was opened with
> > restricted write access solely based on its file operations that it was
> > opened with. This avoids wasting an FMODE_* flag.
> > 
> > def_blk_fops isn't needed to check whether something is a block device
> > checking the inode type is enough for that. And def_blk_fops_restricted
> > can be kept private to the block layer.
> > 
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> 
> I don't think we need def_blk_fops_restricted. If we have BLK_OPEN_WRITE
> file against a bdev with bdev_writes_blocked() == true, we are sure this is
> the handle blocking other writes so we can unblock them in
> bdev_yield_write_access()...

Excellent:

commit e2dd15e4c32ad66d938d35e1acd26375a7f355fb
Author:     Christian Brauner <brauner@xxxxxxxxxx>
AuthorDate: Tue Jan 23 14:26:48 2024 +0100
Commit:     Christian Brauner <brauner@xxxxxxxxxx>
CommitDate: Thu Feb 1 17:13:16 2024 +0100

    block: don't rely on BLK_OPEN_RESTRICT_WRITES when yielding write access

    Make it possible to detected a block device that was opened with
    restricted write access based only on BLK_OPEN_WRITE and
    bdev->bd_writers < 0 so we won't have to claim another FMODE_* flag.

    Link: https://lore.kernel.org/r/20240123-vfs-bdev-file-v2-31-adbd023e19cc@xxxxxxxxxx
    base-commit: 0bd1bf95a554f5f877724c27dbe33d4db0af4d0c
    change-id: 20240129-vfs-bdev-file-bd_inode-385a56c57a51
    Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>

diff --git a/block/bdev.c b/block/bdev.c
index 9be8c3c683ae..0edeb073e4d8 100644
--- a/block/bdev.c
+++ b/block/bdev.c
@@ -799,13 +799,22 @@ static void bdev_claim_write_access(struct block_device *bdev, blk_mode_t mode)
                bdev->bd_writers++;
 }

-static void bdev_yield_write_access(struct block_device *bdev, blk_mode_t mode)
+static void bdev_yield_write_access(struct file *bdev_file, blk_mode_t mode)
 {
+       struct block_device *bdev;
+
        if (bdev_allow_write_mounted)
                return;

+       bdev = file_bdev(bdev_file);
        /* Yield exclusive or shared write access. */
-       if (mode & BLK_OPEN_RESTRICT_WRITES)
+       if (mode & BLK_OPEN_WRITE) {
+               if (bdev_writes_blocked(bdev))
+                       bdev_unblock_writes(bdev);
+               else
+                       bdev->bd_writers--;
+       }
+       if (bdev_file->f_op == &def_blk_fops_restricted)
                bdev_unblock_writes(bdev);
        else if (mode & BLK_OPEN_WRITE)
                bdev->bd_writers--;
@@ -1020,7 +1029,7 @@ void bdev_release(struct file *bdev_file)
                sync_blockdev(bdev);

        mutex_lock(&disk->open_mutex);
-       bdev_yield_write_access(bdev, handle->mode);
+       bdev_yield_write_access(bdev_file, handle->mode);

        if (handle->holder)
                bd_end_claim(bdev, handle->holder);