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

Sweet Tea Dorminy
2022-03-30T16:33:57Z

On 3/30/22 12:03, Omar Sandoval wrote:
> 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.

Since vmalloc allocates whole pages, and kmalloc guarantees alignment to
the allocation size for powers of 2 sizes (and PAGE_SIZE is required to
be a power of 2), I think that adds up to a guarantee of page alignment
both ways?

https://elixir.bootlin.com/linux/v5.17.1/source/include/linux/slab.h#L522 :
kmalloc: "For @size of power of two bytes, the alignment is also
guaranteed to be at least to the size."
https://elixir.bootlin.com/linux/v5.17.1/source/mm/vmalloc.c#L3180:
vmalloc: " Allocate enough pages"...