Re: [PATCH net-next v2 1/4] mm: page_frag: Introduce page_frag_alloc_align()

On Tue, Feb 02, 2021 at 12:48:28PM +0100, Vlastimil Babka wrote:

On 2/2/21 12:36 PM, Ioana Ciornei wrote:

quoted

On Sun, Jan 31, 2021 at 03:44:23PM +0800, Kevin Hao wrote:

quoted

In the current implementation of page_frag_alloc(), it doesn't have
any align guarantee for the returned buffer address. But for some
hardwares they do require the DMA buffer to be aligned correctly,
so we would have to use some workarounds like below if the buffers
allocated by the page_frag_alloc() are used by these hardwares for
DMA.
    buf = page_frag_alloc(really_needed_size + align);
    buf = PTR_ALIGN(buf, align);

These codes seems ugly and would waste a lot of memories if the buffers
are used in a network driver for the TX/RX.

Isn't the memory wasted even with this change?

Yes, but less of it. Not always full amount of align, but up to it. Perhaps even
zero.

Indeed, the worst case is still there but we gain by not allocating the
full 'size + align' all the time.

Thanks.

quoted

I am not familiar with the frag allocator so I might be missing
something, but from what I understood each page_frag_cache keeps only
the offset inside the current page being allocated, offset which you
ALIGN_DOWN() to match the alignment requirement. I don't see how that
memory between the non-aligned and aligned offset is going to be used
again before the entire page is freed.

True, thath's how page_frag is designed. The align amounts would be most likely
too small to be usable anyway.