Re: [PATCH] btrfs: use IOMAP_DIO_BOUNCE flag for direct reads

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 28, 2026 at 06:58:16PM +0930, Qu Wenruo wrote:
> Similar to direct writes, if the inode requires data checksum, we should
> use a stable buffer for IO, or even for direct reads an unstable buffer
> (e.g. the buffer is being modified during the direct read) can lead to
> checksum mismatch and even cause read failure.
> 
> To address the potential problems of unstable dio read buffers, use
> IOMAP_DIO_BOUNCE for dio reads if the inode requires data checksum.
> 
> This will bring a small performance drop, around 7% for my benchmark,
> which is definitely observable on modern NVME SSDs, but the overhead is
> still much smaller compared to data checksum:

Please take a look at the "lazy bounce buffering for checksummed reads"
series I just sent v2 of.

For one this removes the read-side IOMAP_DIO_BOUNCE support you are
using here, because it had problems.  The alternative is iomap-based
and better at building large, aligned read bios which matter for HDD.
It also introduces the concept of lazy bouncing where it only bounce
buffers after a checksum failure happens by default, which means you
only pay an overhead for misbehaving applications.  And unlike for
writes where they do exist in real life, I don't know any that modify
the buffer they read into.