On Wed, 18 Mar 2026, Keith Busch wrote:
> On Wed, Mar 18, 2026 at 06:40:52PM +0100, Mikulas Patocka wrote:
> > On Wed, 18 Mar 2026, Keith Busch wrote:
> >
> > > But even then, the only thing I know of that really wants this has an
> > > offset that straddles two pages per block, so I never need more than 2
> > > segments, and the inline scatterlist has four. There's just currently no
> > > way for the block layer to report a max-segments-per-block limit, so I'm
> > > including this patch to be consistent with the reportable limits.
> >
> > Can userspace use preadv/pwritev with many small iovecs on a file opened
> > for direct I/O?
>
> Yes, I have several patches that were accepted last year to block and
> iomap that align direct-io memory requirements to the backing device's
> dma constraints. You can do dword aligned and sized vectored IO to a
> block device today if the block limits say its okay.
So, we must support worst-case scenario - "cc->sector_size / 4" segments.
sector_size can be up to 4096, so there may be up to 1024 segments. Each
segment consumes sizeof(struct scatterlist) bytes - that's 32 bytes. So,
we allocate up to 32768 bytes.
kmalloc with size greater than 32768 is unreliable - so we are just on
this limit.
Mikulas
