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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 2 May 2022 10:48:49 -0700
Message-ID: <CAHbLzkpF4zedBmipjX8Zy5F=Fffez+xgxTAvveaz1nRHb9Wg_Q@mail.gmail.com>
Subject: Re: [PATCH 1/2] MM: handle THP in swap_*page_fs()
To:     NeilBrown <neilb@suse.de>, Huang Ying <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Christoph Hellwig <hch@lst.de>,
        Miaohe Lin <linmiaohe@huawei.com>, linux-nfs@vger.kernel.org,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sun, May 1, 2022 at 9:23 PM NeilBrown <neilb@suse.de> wrote:
>
> On Sat, 30 Apr 2022, Yang Shi wrote:
> > On Thu, Apr 28, 2022 at 5:44 PM NeilBrown <neilb@suse.de> wrote:
> > >
> > > Pages passed to swap_readpage()/swap_writepage() are not necessarily all
> > > the same size - there may be transparent-huge-pages involves.
> > >
> > > The BIO paths of swap_*page() handle this correctly, but the SWP_FS_OPS
> > > path does not.
> > >
> > > So we need to use thp_size() to find the size, not just assume
> > > PAGE_SIZE, and we need to track the total length of the request, not
> > > just assume it is "page * PAGE_SIZE".
> >
> > Swap-over-nfs doesn't support THP swap IIUC. So SWP_FS_OPS should not
> > see THP at all. But I agree to remove the assumption about page size
> > in this path.
>
> Can you help me understand this please.  How would the swap code know
> that swap-over-NFS doesn't support THP swap?  There is no reason that
> NFS wouldn't be able to handle 2MB writes.  Even 1GB should work though
> NFS would have to split into several smaller WRITE requests.

AFAICT, THP swap is only supported on non-rotate block devices, for
example, SSD, PMEM, etc. IIRC, the swap device has to support the
cluster in order to swap THP. The cluster is only supported by
non-rotate block devices.

Looped Ying in, who is the author of THP swap.

>
> Thanks,
> NeilBrown
>
>
> >
> > >
> > > Reported-by: Miaohe Lin <linmiaohe@huawei.com>
> > > Signed-off-by: NeilBrown <neilb@suse.de>
> > > ---
> > >  mm/page_io.c |   23 +++++++++++++----------
> > >  1 file changed, 13 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/mm/page_io.c b/mm/page_io.c
> > > index c132511f521c..d636a3531cad 100644
> > > --- a/mm/page_io.c
> > > +++ b/mm/page_io.c
> > > @@ -239,6 +239,7 @@ struct swap_iocb {
> > >         struct kiocb            iocb;
> > >         struct bio_vec          bvec[SWAP_CLUSTER_MAX];
> > >         int                     pages;
> > > +       int                     len;
> > >  };
> > >  static mempool_t *sio_pool;
> > >
> > > @@ -261,7 +262,7 @@ static void sio_write_complete(struct kiocb *iocb, long ret)
> > >         struct page *page = sio->bvec[0].bv_page;
> > >         int p;
> > >
> > > -       if (ret != PAGE_SIZE * sio->pages) {
> > > +       if (ret != sio->len) {
> > >                 /*
> > >                  * In the case of swap-over-nfs, this can be a
> > >                  * temporary failure if the system has limited
> > > @@ -301,7 +302,7 @@ static int swap_writepage_fs(struct page *page, struct writeback_control *wbc)
> > >                 sio = *wbc->swap_plug;
> > >         if (sio) {
> > >                 if (sio->iocb.ki_filp != swap_file ||
> > > -                   sio->iocb.ki_pos + sio->pages * PAGE_SIZE != pos) {
> > > +                   sio->iocb.ki_pos + sio->len != pos) {
> > >                         swap_write_unplug(sio);
> > >                         sio = NULL;
> > >                 }
> > > @@ -312,10 +313,12 @@ static int swap_writepage_fs(struct page *page, struct writeback_control *wbc)
> > >                 sio->iocb.ki_complete = sio_write_complete;
> > >                 sio->iocb.ki_pos = pos;
> > >                 sio->pages = 0;
> > > +               sio->len = 0;
> > >         }
> > >         sio->bvec[sio->pages].bv_page = page;
> > > -       sio->bvec[sio->pages].bv_len = PAGE_SIZE;
> > > +       sio->bvec[sio->pages].bv_len = thp_size(page);
> > >         sio->bvec[sio->pages].bv_offset = 0;
> > > +       sio->len += thp_size(page);
> > >         sio->pages += 1;
> > >         if (sio->pages == ARRAY_SIZE(sio->bvec) || !wbc->swap_plug) {
> > >                 swap_write_unplug(sio);
> > > @@ -371,8 +374,7 @@ void swap_write_unplug(struct swap_iocb *sio)
> > >         struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
> > >         int ret;
> > >
> > > -       iov_iter_bvec(&from, WRITE, sio->bvec, sio->pages,
> > > -                     PAGE_SIZE * sio->pages);
> > > +       iov_iter_bvec(&from, WRITE, sio->bvec, sio->pages, sio->len);
> > >         ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> > >         if (ret != -EIOCBQUEUED)
> > >                 sio_write_complete(&sio->iocb, ret);
> > > @@ -383,7 +385,7 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
> > >         struct swap_iocb *sio = container_of(iocb, struct swap_iocb, iocb);
> > >         int p;
> > >
> > > -       if (ret == PAGE_SIZE * sio->pages) {
> > > +       if (ret == sio->len) {
> > >                 for (p = 0; p < sio->pages; p++) {
> > >                         struct page *page = sio->bvec[p].bv_page;
> > >
> > > @@ -415,7 +417,7 @@ static void swap_readpage_fs(struct page *page,
> > >                 sio = *plug;
> > >         if (sio) {
> > >                 if (sio->iocb.ki_filp != sis->swap_file ||
> > > -                   sio->iocb.ki_pos + sio->pages * PAGE_SIZE != pos) {
> > > +                   sio->iocb.ki_pos + sio->len != pos) {
> > >                         swap_read_unplug(sio);
> > >                         sio = NULL;
> > >                 }
> > > @@ -426,10 +428,12 @@ static void swap_readpage_fs(struct page *page,
> > >                 sio->iocb.ki_pos = pos;
> > >                 sio->iocb.ki_complete = sio_read_complete;
> > >                 sio->pages = 0;
> > > +               sio->len = 0;
> > >         }
> > >         sio->bvec[sio->pages].bv_page = page;
> > > -       sio->bvec[sio->pages].bv_len = PAGE_SIZE;
> > > +       sio->bvec[sio->pages].bv_len = thp_size(page);
> > >         sio->bvec[sio->pages].bv_offset = 0;
> > > +       sio->len += thp_size(page);
> > >         sio->pages += 1;
> > >         if (sio->pages == ARRAY_SIZE(sio->bvec) || !plug) {
> > >                 swap_read_unplug(sio);
> > > @@ -521,8 +525,7 @@ void __swap_read_unplug(struct swap_iocb *sio)
> > >         struct address_space *mapping = sio->iocb.ki_filp->f_mapping;
> > >         int ret;
> > >
> > > -       iov_iter_bvec(&from, READ, sio->bvec, sio->pages,
> > > -                     PAGE_SIZE * sio->pages);
> > > +       iov_iter_bvec(&from, READ, sio->bvec, sio->pages, sio->len);
> > >         ret = mapping->a_ops->swap_rw(&sio->iocb, &from);
> > >         if (ret != -EIOCBQUEUED)
> > >                 sio_read_complete(&sio->iocb, ret);
> > >
> > >
> > >
> >