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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/8/31 16:45, Christoph Hellwig 写道:
> 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.

Thanks for pointing to that series.

However it only shows up in lore, but not in my inbox at all, and I have 
no idea why.

And lore is never a good way to review patches, nor easy to reply.

> 
> 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.

In that case I'm afraid btrfs may need some way to integrate the 
checksum handling into iomap.
As the btrfs has a very internal handling for repairing each block.

>  And unlike for
> writes where they do exist in real life, I don't know any that modify
> the buffer they read into.

Yep, and that's why we didn't put too much attention to dio reads until
the dio writes bounce is implemented.

Thanks,
Qu