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);
Thanks! :)
> + sio->bvec[sio->pages].bv_offset = 0;
...
> .
>
