On Thu, Oct 18, 2018 at 05:22:19PM +0200, Christoph Hellwig wrote:
> On Thu, Oct 18, 2018 at 08:11:23AM -0700, Matthew Wilcox wrote:
> > On Thu, Oct 18, 2018 at 04:42:07PM +0200, Christoph Hellwig wrote:
> > > This all seems quite complicated.
> > >
> > > I think the interface we'd want is more one that has a little
> > > cache of a single page in the queue, and a little bitmap which
> > > sub-page size blocks of it are used.
> > >
> > > Something like (pseudo code minus locking):
> > >
> > > void *blk_alloc_sector_buffer(struct block_device *bdev, gfp_t gfp)
> > > {
> > > unsigned block_size = block_size(bdev);
> > >
> > > if (blocksize >= PAGE_SIZE)
> > > return (void *)__get_free_pages(gfp, get_order(blocksize));
> > >
> > > if (bdev->fragment_cache_page) {
> > > [ <find fragment in bdev->fragment_cache_page using
> > > e.g. bitmap and return if found]
> > > }
> > >
> > > bdev->fragment_cache_page = (void *)__get_free_page(gfp);
> > > goto find_again;
> > > }
> >
> > This looks a lot like page_frag_alloc() except I think page_frag_alloc()
> > may be more efficient.
>
> Oh, nice. Sounds like XFS should just use page_frag_alloc. I'll give
> it a spin.
XFS or other fs can use page_frag_alloc() directly, seems not necessary to
introduce this change in block layer any more given 512-aligned buffer
should be fine everywhere.
The only benefit to make it as block helper is that the offset or size
can be checked with q->dma_alignment.
Dave/Jens, do you think which way is better? Put allocation as block
helper or fs uses page_frag_alloc() directly for allocating 512*N-byte
buffer(total size is less than PAGE_SIZE)?
Thanks,
Ming
