On 2/2/21 12:36 PM, Ioana Ciornei wrote: > On Sun, Jan 31, 2021 at 03:44:23PM +0800, Kevin Hao wrote: >> 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. > 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.
