On Mon, 18 Apr 2022, Miaohe Lin wrote:
> On 2022/3/30 7:49, NeilBrown wrote:
> > swap_writepage() is given one page at a time, but may be called repeatedly
> > in succession.
> > For block-device swapspace, the blk_plug functionality allows the
> > multiple pages to be combined together at lower layers.
> > That cannot be used for SWP_FS_OPS as blk_plug may not exist - it is
> > only active when CONFIG_BLOCK=y. Consequently all swap reads over NFS
> > are single page reads.
> >
> > With this patch we pass a pointer-to-pointer via the wbc.
> > swap_writepage can store state between calls - much like the pointer
> > passed explicitly to swap_readpage. After calling swap_writepage() some
> > number of times, the state will be passed to swap_write_unplug() which
> > can submit the combined request.
> >
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ...
> >
> > static int swap_writepage_fs(struct page *page, struct writeback_control *wbc)
> > {
> > - struct swap_iocb *sio;
> > + struct swap_iocb *sio = NULL;
> > struct swap_info_struct *sis = page_swap_info(page);
> > struct file *swap_file = sis->swap_file;
> > - struct address_space *mapping = swap_file->f_mapping;
> > - struct iov_iter from;
> > - int ret;
> > + loff_t pos = page_file_offset(page);
> >
> > set_page_writeback(page);
> > unlock_page(page);
> > - sio = mempool_alloc(sio_pool, GFP_NOIO);
> > - init_sync_kiocb(&sio->iocb, swap_file);
> > - sio->iocb.ki_complete = sio_write_complete;
> > - sio->iocb.ki_pos = page_file_offset(page);
> > - sio->bvec[0].bv_page = page;
> > - sio->bvec[0].bv_len = PAGE_SIZE;
> > - sio->bvec[0].bv_offset = 0;
> > - iov_iter_bvec(&from, WRITE, &sio->bvec[0], 1, PAGE_SIZE);
> > - ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> > - if (ret != -EIOCBQUEUED)
> > - sio_write_complete(&sio->iocb, ret);
> > - return ret;
> > + if (wbc->swap_plug)
> > + sio = *wbc->swap_plug;
> > + if (sio) {
> > + if (sio->iocb.ki_filp != swap_file ||
> > + sio->iocb.ki_pos + sio->pages * PAGE_SIZE != pos) {
> > + swap_write_unplug(sio);
> > + sio = NULL;
> > + }
> > + }
> > + if (!sio) {
> > + sio = mempool_alloc(sio_pool, GFP_NOIO);
> > + init_sync_kiocb(&sio->iocb, swap_file);
> > + sio->iocb.ki_complete = sio_write_complete;
> > + sio->iocb.ki_pos = pos;
> > + sio->pages = 0;
> > + }
> > + sio->bvec[sio->pages].bv_page = page;
> > + sio->bvec[sio->pages].bv_len = PAGE_SIZE;
>
> Many thanks for your patch. And sorry for late responding and newbie question. Does swap_writepage_fs
> support transhuge page now? We could come across transhuge page here. But bv_len == PAGE_SIZE and pages
> == 1 is assumed here. Do we need something like below:
>
> sio->bvec[sio->pages].bv_len = thp_size(page);
> sio->pages += thp_nr_pages(page);
Yes, that probably makes sense. I'll have a closer look and maybe
resend later this week.
Thanks,
NeilBrown
>
> Thanks! :)
>
> > + sio->bvec[sio->pages].bv_offset = 0;
> ...
> > .
> >
>
>
