On Tue, Feb 02, 2021 at 12:48:28PM +0100, Vlastimil Babka wrote: > 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. Indeed, the worst case is still there but we gain by not allocating the full 'size + align' all the time. Thanks. > > > 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.
