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SubjectRe: [PATCH 03/18] MM: use ->swap_rw for reads from SWP_FS_OPS swap-space
> +int sio_pool_init(void)
> +{
> + if (!sio_pool)
> + sio_pool = mempool_create_kmalloc_pool(
> + SWAP_CLUSTER_MAX, sizeof(struct swap_iocb));

I can't see anything serializing access here, so we'll need a lock or
cmpxchg dance.

> + if (sio_pool)
> + return 0;
> + else
> + return -ENOMEM;

Nit: This would flow much nicer as:

if (!sio_pool)
return -ENOMEM;
return 0;

> int swap_readpage(struct page *page, bool synchronous)
> {
> struct bio *bio;
> @@ -378,13 +412,25 @@ int swap_readpage(struct page *page, bool synchronous)
> }
>
> if (data_race(sis->flags & SWP_FS_OPS)) {
> - //struct file *swap_file = sis->swap_file;
> - //struct address_space *mapping = swap_file->f_mapping;

This should not be left by the previous patch. In fact I suspect the
part of the previous patch that adds ->swap_rw should probably be folded
into this patch.

> + struct file *swap_file = sis->swap_file;
> + struct address_space *mapping = swap_file->f_mapping;
> + struct iov_iter from;
> + struct swap_iocb *sio;
> + loff_t pos = page_file_offset(page);
> +
> + sio = mempool_alloc(sio_pool, GFP_KERNEL);
> + init_sync_kiocb(&sio->iocb, swap_file);
> + sio->iocb.ki_pos = pos;
> + sio->iocb.ki_complete = sio_read_complete;
> + sio->bvec.bv_page = page;
> + sio->bvec.bv_len = PAGE_SIZE;
> + sio->bvec.bv_offset = 0;
> +
> + iov_iter_bvec(&from, READ, &sio->bvec, 1, PAGE_SIZE);
> + ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> + if (ret != -EIOCBQUEUED)
> + sio_read_complete(&sio->iocb, ret);
>
> goto out;

I'd be tempted to split the SWP_FS_OPS into a helper to keep the
code tidy.

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