| Message ID | 5d52220b-177d-72d4-7825-dbe6cbf8722f@inwind.it (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] btrfs : avoid O_DIRECT when the file is protected by CSUM | expand |
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 0e41459b8de6..af73157e8200 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -2018,7 +2018,12 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb, if (sync) atomic_inc(&BTRFS_I(inode)->sync_writers); - if (iocb->ki_flags & IOCB_DIRECT) + /* + * O_DIRECT doesn't play well with CSUM, so allow the O_DIRECT + * only if the file is marked BTRFS_INODE_NODATASUM + */ + if (iocb->ki_flags & IOCB_DIRECT && + (BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM)) num_written = btrfs_direct_write(iocb, from); else num_written = btrfs_buffered_write(iocb, from);
Dear all, this is an RFC. I made very few test, so use at your risk. In the past there were several reports [1][2] about corruption when O_DIRECT is used. Every time the analysis was the same: it is too difficult to sync the checksum with the data when O_DIRECT is used. This aim of this patch is to avoid the corruption disabling the O_DIRECT write when the file is NOT marked as NODATACSUM. In other words, O_DIRECT is honored only for the file not protected by CSUM: O_DIRECT ^ DATASUM The user-space is not informed that O_DIRECT is not honored. On the best of my knowledge, ZFS does the same thing. It is not a regression: today an O_DIRECT file update may compromise the checksum calculation. So may be the file content is correct, but the checksum no.This prevent to read the file. In [2] there is a test program which trigger the problem. With this patch the program doesn't trigger the problem. Open question: - does the kernel return an error when a file is opened with O_DIRECT and the file has the checksum ? - does make sense to have a mount option to select different behaviours: 1) let the kernel to behave as today (O_DIRECT is allowed even at risk of having some form of corruption) 2) let the kernel to ignore O_DIRECT if the file is protected by checksum 3) let the kernel to return error when O_DIRECT is used with file with checksum ? Comments are welcome. BR G.Baroncelli [1] https://lore.kernel.org/linux-btrfs/1ad3962943592e9a60f88aecdb493f368c70bbe1.camel@infradead.org/#r [2] https://lore.kernel.org/linux-btrfs/cf8a733f-2c9d-7ffe-e865-4c13d99dfb60@libero.it/ ----- -----------