Re: [PATCH v14 5/7] btrfs: send: allocate send buffer with alloc_page() and vmap() for v2

Omar Sandoval
2022-03-30T16:03:22Z

On Thu, Mar 24, 2022 at 01:53:20PM -0400, Sweet Tea Dorminy wrote:
>
>
> On 3/17/22 13:25, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@fb.com>
> >
> > For encoded writes, we need the raw pages for reading compressed data
> > directly via a bio.
> Perhaps:
> "For encoded writes, the existing btrfs_encoded_read*() functions expect a
> list of raw pages."
>
> I think it would be a better to continue just vmalloc'ing a large continuous
> buffer and translating each page in the buffer into its raw page with
> something like is_vmalloc_addr(data) ? vmalloc_to_page(data) :
> virt_to_page(data). Vmalloc can request a higher-order allocation, which
> probably doesn't matter but might slightly improve memory locality. And in
> terms of readability, I somewhat like the elegance of having a single
> identical kvmalloc call to allocate and send_buf in both cases, even if we
> do need to initialize the page list for some v2 commands.

I like this, but are we guaranteed that kvmalloc() will return a
page-aligned buffer? It seems reasonable to me that it would for
allocations of at least one page, but I can't find that written down
anywhere.