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Message-ID: <CAGsJ_4w2k=704mgtQu97y5Qpidc-x+ZBmBXCytkzdcasfAaG0w@mail.gmail.com>
Date: Wed, 4 Sep 2024 19:17:17 +1200
From: Barry Song <21cnbao@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: usamaarif642@...il.com, akpm@...ux-foundation.org,
chengming.zhou@...ux.dev, david@...hat.com, hannes@...xchg.org,
hughd@...gle.com, kernel-team@...a.com, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, nphamcs@...il.com, shakeel.butt@...ux.dev,
willy@...radead.org, ying.huang@...el.com, hanchuanhua@...o.com
Subject: Re: [PATCH v4 1/2] mm: store zero pages to be swapped out in a bitmap
On Wed, Sep 4, 2024 at 7:12 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> [..]
> > > @@ -426,6 +515,26 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
> > > mempool_free(sio, sio_pool);
> > > }
> > >
> > > +static bool swap_read_folio_zeromap(struct folio *folio)
> > > +{
> > > + unsigned int idx = swap_zeromap_folio_test(folio);
> > > +
> > > + if (idx == 0)
> > > + return false;
> > > +
> > > + /*
> > > + * Swapping in a large folio that is partially in the zeromap is not
> > > + * currently handled. Return true without marking the folio uptodate so
> > > + * that an IO error is emitted (e.g. do_swap_page() will sigbus).
> > > + */
> > > + if (WARN_ON_ONCE(idx < folio_nr_pages(folio)))
> > > + return true;
> >
> > Hi Usama, Yosry,
> >
> > I feel the warning is wrong as we could have the case where idx==0
> > is not zeromap but idx=1 is zeromap. idx == 0 doesn't necessarily
> > mean we should return false.
>
> Good catch. Yeah if idx == 0 is not in the zeromap but other indices
> are we will mistakenly read the entire folio from swap.
>
> >
> > What about the below change which both fixes the warning and unblocks
> > large folios swap-in?
>
> But I don't see how that unblocks the large folios swap-in work? We
> still need to actually handle the case where a large folio being
> swapped in is partially in the zeromap. Right now we warn and unlock
> the folio without calling folio_mark_uptodate(), which emits an IO
> error.
I placed this in mm/swap.h so that during swap-in, it can filter out any large
folios where swap_zeromap_entries_count() is greater than 0 and less than
nr.
I believe this case would be quite rare, as it can only occur when some small
folios that are swapped out happen to have contiguous and aligned swap
slots.
>
> >
> > diff --git a/mm/page_io.c b/mm/page_io.c
> > index 4bc77d1c6bfa..7d7ff7064e2b 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -226,26 +226,6 @@ static void swap_zeromap_folio_clear(struct folio *folio)
> > }
> > }
> >
> > -/*
> > - * Return the index of the first subpage which is not zero-filled
> > - * according to swap_info_struct->zeromap.
> > - * If all pages are zero-filled according to zeromap, it will return
> > - * folio_nr_pages(folio).
> > - */
> > -static unsigned int swap_zeromap_folio_test(struct folio *folio)
> > -{
> > - struct swap_info_struct *sis = swp_swap_info(folio->swap);
> > - swp_entry_t entry;
> > - unsigned int i;
> > -
> > - for (i = 0; i < folio_nr_pages(folio); i++) {
> > - entry = page_swap_entry(folio_page(folio, i));
> > - if (!test_bit(swp_offset(entry), sis->zeromap))
> > - return i;
> > - }
> > - return i;
> > -}
> > -
> > /*
> > * We may have stale swap cache pages in memory: notice
> > * them here and get rid of the unnecessary final write.
> > @@ -524,9 +504,10 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
> >
> > static bool swap_read_folio_zeromap(struct folio *folio)
> > {
> > - unsigned int idx = swap_zeromap_folio_test(folio);
> > + unsigned int nr_pages = folio_nr_pages(folio);
> > + unsigned int nr = swap_zeromap_entries_count(folio->swap, nr_pages);
> >
> > - if (idx == 0)
> > + if (nr == 0)
> > return false;
> >
> > /*
> > @@ -534,7 +515,7 @@ static bool swap_read_folio_zeromap(struct folio *folio)
> > * currently handled. Return true without marking the folio uptodate so
> > * that an IO error is emitted (e.g. do_swap_page() will sigbus).
> > */
> > - if (WARN_ON_ONCE(idx < folio_nr_pages(folio)))
> > + if (WARN_ON_ONCE(nr < nr_pages))
> > return true;
> >
> > folio_zero_range(folio, 0, folio_size(folio));
> > diff --git a/mm/swap.h b/mm/swap.h
> > index f8711ff82f84..2d59e9d89e95 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -80,6 +80,32 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
> > {
> > return swp_swap_info(folio->swap)->flags;
> > }
> > +
> > +/*
> > + * Return the number of entries which are zero-filled according to
> > + * swap_info_struct->zeromap. It isn't precise if the return value
> > + * is larger than 0 and smaller than nr to avoid extra iterations,
> > + * In this case, it means entries haven't consistent zeromap.
> > + */
> > +static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry, int nr)
> > +{
> > + struct swap_info_struct *sis = swp_swap_info(entry);
> > + unsigned long offset = swp_offset(entry);
> > + unsigned int type = swp_type(entry);
> > + unsigned int n = 0;
> > +
> > + for (int i = 0; i < nr; i++) {
> > + entry = swp_entry(type, offset + i);
> > + if (test_bit(offset + i, sis->zeromap)) {
> > + if (i != n)
> > + return i;
> > + n++;
> > + }
> > + }
> > +
> > + return n;
> > +}
> > +
> > #else /* CONFIG_SWAP */
> > struct swap_iocb;
> > static inline void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
> > @@ -171,6 +197,11 @@ static inline unsigned int folio_swap_flags(struct folio *folio)
> > {
> > return 0;
> > }
> > +
> > +static inline unsigned int swap_zeromap_entries_count(swp_entry_t entry, int nr)
> > +{
> > + return 0;
> > +}
> > #endif /* CONFIG_SWAP */
> >
> > #endif /* _MM_SWAP_H */
> >
> > > +
> > > + folio_zero_fill(folio);
> > > + folio_mark_uptodate(folio);
> > > + return true;
> > > +}
> > > +
> > > static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
> > > {
> > > struct swap_info_struct *sis = swp_swap_info(folio->swap);
> > > @@ -515,8 +624,9 @@ void swap_read_folio(struct folio *folio, bool synchronous,
> > > psi_memstall_enter(&pflags);
> > > }
> > > delayacct_swapin_start();
> > > -
> > > - if (zswap_load(folio)) {
> > > + if (swap_read_folio_zeromap(folio)) {
> > > + folio_unlock(folio);
> > > + } else if (zswap_load(folio)) {
> > > folio_mark_uptodate(folio);
> > > folio_unlock(folio);
> > > } else if (data_race(sis->flags & SWP_FS_OPS)) {
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index f1e559e216bd..48d8dca0b94b 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -453,6 +453,8 @@ static unsigned int cluster_list_del_first(struct swap_cluster_list *list,
> > > static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> > > unsigned int idx)
> > > {
> > > + unsigned int i;
> > > +
> > > /*
> > > * If scan_swap_map_slots() can't find a free cluster, it will check
> > > * si->swap_map directly. To make sure the discarding cluster isn't
> > > @@ -461,6 +463,13 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
> > > */
> > > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > > SWAP_MAP_BAD, SWAPFILE_CLUSTER);
> > > + /*
> > > + * zeromap can see updates from concurrent swap_writepage() and swap_read_folio()
> > > + * call on other slots, hence use atomic clear_bit for zeromap instead of the
> > > + * non-atomic bitmap_clear.
> > > + */
> > > + for (i = 0; i < SWAPFILE_CLUSTER; i++)
> > > + clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
> > >
> > > cluster_list_add_tail(&si->discard_clusters, si->cluster_info, idx);
> > >
> > > @@ -482,7 +491,7 @@ static void __free_cluster(struct swap_info_struct *si, unsigned long idx)
> > > static void swap_do_scheduled_discard(struct swap_info_struct *si)
> > > {
> > > struct swap_cluster_info *info, *ci;
> > > - unsigned int idx;
> > > + unsigned int idx, i;
> > >
> > > info = si->cluster_info;
> > >
> > > @@ -498,6 +507,8 @@ static void swap_do_scheduled_discard(struct swap_info_struct *si)
> > > __free_cluster(si, idx);
> > > memset(si->swap_map + idx * SWAPFILE_CLUSTER,
> > > 0, SWAPFILE_CLUSTER);
> > > + for (i = 0; i < SWAPFILE_CLUSTER; i++)
> > > + clear_bit(idx * SWAPFILE_CLUSTER + i, si->zeromap);
> > > unlock_cluster(ci);
> > > }
> > > }
> > > @@ -1059,9 +1070,12 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
> > > {
> > > unsigned long offset = idx * SWAPFILE_CLUSTER;
> > > struct swap_cluster_info *ci;
> > > + unsigned int i;
> > >
> > > ci = lock_cluster(si, offset);
> > > memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER);
> > > + for (i = 0; i < SWAPFILE_CLUSTER; i++)
> > > + clear_bit(offset + i, si->zeromap);
> > > cluster_set_count_flag(ci, 0, 0);
> > > free_cluster(si, idx);
> > > unlock_cluster(ci);
> > > @@ -1336,6 +1350,7 @@ static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
> > > count = p->swap_map[offset];
> > > VM_BUG_ON(count != SWAP_HAS_CACHE);
> > > p->swap_map[offset] = 0;
> > > + clear_bit(offset, p->zeromap);
> > > dec_cluster_info_page(p, p->cluster_info, offset);
> > > unlock_cluster(ci);
> > >
> > > @@ -2597,6 +2612,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> > > free_percpu(p->cluster_next_cpu);
> > > p->cluster_next_cpu = NULL;
> > > vfree(swap_map);
> > > + bitmap_free(p->zeromap);
> > > kvfree(cluster_info);
> > > /* Destroy swap account information */
> > > swap_cgroup_swapoff(p->type);
> > > @@ -3123,6 +3139,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> > > goto bad_swap_unlock_inode;
> > > }
> > >
> > > + p->zeromap = bitmap_zalloc(maxpages, GFP_KERNEL);
> > > + if (!p->zeromap) {
> > > + error = -ENOMEM;
> > > + goto bad_swap_unlock_inode;
> > > + }
> > > +
> > > if (p->bdev && bdev_stable_writes(p->bdev))
> > > p->flags |= SWP_STABLE_WRITES;
> > >
> > > --
> > > 2.43.0
> > >
> > >
Thanks
barry
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