Re: [RFC PATCH v3 2/5] mm: swap: introduce swap_nr_free() for batched swap_free()

[Chuanhua Han]

Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年3月14日周四 21:43写道：
>
> On 14/03/2024 13:12, Chuanhua Han wrote:
> > Ryan Roberts <ryan.roberts@xxxxxxx> 于2024年3月12日周二 02:51写道：
> >>
> >> On 04/03/2024 08:13, Barry Song wrote:
> >>> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >>>
> >>> While swapping in a large folio, we need to free swaps related to the whole
> >>> folio. To avoid frequently acquiring and releasing swap locks, it is better
> >>> to introduce an API for batched free.
> >>>
> >>> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> >>> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>> ---
> >>>  include/linux/swap.h |  6 ++++++
> >>>  mm/swapfile.c        | 35 +++++++++++++++++++++++++++++++++++
> >>>  2 files changed, 41 insertions(+)
> >>>
> >>> diff --git a/include/linux/swap.h b/include/linux/swap.h
> >>> index 2955f7a78d8d..d6ab27929458 100644
> >>> --- a/include/linux/swap.h
> >>> +++ b/include/linux/swap.h
> >>> @@ -481,6 +481,7 @@ extern void swap_shmem_alloc(swp_entry_t);
> >>>  extern int swap_duplicate(swp_entry_t);
> >>>  extern int swapcache_prepare(swp_entry_t);
> >>>  extern void swap_free(swp_entry_t);
> >>> +extern void swap_nr_free(swp_entry_t entry, int nr_pages);
> >>
> >> nit: In my swap-out v4 series, I've created a batched version of
> >> free_swap_and_cache() and called it free_swap_and_cache_nr(). Perhaps it is
> >> preferable to align the naming schemes - i.e. call this swap_free_nr() Your
> >> scheme doesn't really work when applied to free_swap_and_cache().
> > Thanks for your suggestions, and for the next version, we'll see which
> > package is more appropriate!
> >>
> >>>  extern void swapcache_free_entries(swp_entry_t *entries, int n);
> >>>  extern int free_swap_and_cache(swp_entry_t);
> >>>  int swap_type_of(dev_t device, sector_t offset);
> >>> @@ -561,6 +562,11 @@ static inline void swap_free(swp_entry_t swp)
> >>>  {
> >>>  }
> >>>
> >>> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> >>> +{
> >>> +
> >>> +}
> >>> +
> >>>  static inline void put_swap_folio(struct folio *folio, swp_entry_t swp)
> >>>  {
> >>>  }
> >>> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >>> index 3f594be83b58..244106998a69 100644
> >>> --- a/mm/swapfile.c
> >>> +++ b/mm/swapfile.c
> >>> @@ -1341,6 +1341,41 @@ void swap_free(swp_entry_t entry)
> >>>               __swap_entry_free(p, entry);
> >>>  }
> >>>
> >>> +/*
> >>> + * Called after swapping in a large folio, batched free swap entries
> >>> + * for this large folio, entry should be for the first subpage and
> >>> + * its offset is aligned with nr_pages
> >>> + */
> >>> +void swap_nr_free(swp_entry_t entry, int nr_pages)
> >>> +{
> >>> +     int i;
> >>> +     struct swap_cluster_info *ci;
> >>> +     struct swap_info_struct *p;
> >>> +     unsigned type = swp_type(entry);
> >>
> >> nit: checkpatch.py will complain about bare "unsigned", preferring "unsigned
> >> int" or at least it did for me when I did something similar in my swap-out patch
> >> set.
> > Gee, thanks for pointing that out!
> >>
> >>> +     unsigned long offset = swp_offset(entry);
> >>> +     DECLARE_BITMAP(usage, SWAPFILE_CLUSTER) = { 0 };
> >>
> >> I don't love this, as it could blow the stack if SWAPFILE_CLUSTER ever
> >> increases. But the only other way I can think of is to explicitly loop over
> >> fixed size chunks, and that's not much better.
> > Is it possible to save kernel stack better by using bit_map here?  If
> > SWAPFILE_CLUSTER=512, we consume only (512/64)*8= 64 bytes.
>
> I'm not sure I've understood what you are saying? You're already using
> DECLARE_BITMAP(), so its already consuming 64 bytes if SWAPFILE_CLUSTER=512, no?
>
> I actually did a bad job of trying to express a couple of different points:
>
> - Are there any configurations today where SWAPFILE_CLUSTER > 512? I'm not sure.
> Certainly not for arm64, but not sure about other architectures. For example if
> an arch had 64K pages with 8192 entries per THP and supports SWAP_THP, that's 1K
> for the bitmap, which is now looking pretty big for the stack.
I agree with you.The current bit_map grows linearly with the
SWAPFILE_CLUSTER, which may cause the kernel stack to swell.
I need to think of a way to save more memory .
>
> - Would it be better to decouple stack usage from SWAPFILE_CLUSTER and instead
> define a fixed stack size (e.g. 64 bytes -> 512 entries). Then free the range of
> entries in batches no bigger than this size. This approach could also allow
> removing the constraint that the range has to be aligned and fit in a single
> cluster. Personally I think an approach like this would be much more robust, in
> return for a tiny bit more complexity.
Because we cannot determine how many swap entries a cluster has in an
architecture or a configuration, we do not know how large the variable
needs to be defined？
>
> >>
> >>> +
> >>> +     /* all swap entries are within a cluster for mTHP */
> >>> +     VM_BUG_ON(offset % SWAPFILE_CLUSTER + nr_pages > SWAPFILE_CLUSTER);
> >>> +
> >>> +     if (nr_pages == 1) {
> >>> +             swap_free(entry);
> >>> +             return;
> >>> +     }
> >>> +
> >>> +     p = _swap_info_get(entry);
> >>
> >> You need to handle this returning NULL, like swap_free() does.
> > Yes, you're right! We did forget to judge NULL here.
> >>
> >>> +
> >>> +     ci = lock_cluster(p, offset);
> >>
> >> The existing swap_free() calls lock_cluster_or_swap_info(). So if swap is backed
> >> by rotating media, and clusters are not in use, it will lock the whole swap
> >> info. But your new version only calls lock_cluster() which won't lock anything
> >> if clusters are not in use. So I think this is a locking bug.
> > Again, you're right, it's bug!
> >>
> >>> +     for (i = 0; i < nr_pages; i++) {
> >>> +             if (__swap_entry_free_locked(p, offset + i, 1))
> >>> +                     __bitmap_set(usage, i, 1);
> >>> +     }
> >>> +     unlock_cluster(ci);
> >>> +
> >>> +     for_each_clear_bit(i, usage, nr_pages)
> >>> +             free_swap_slot(swp_entry(type, offset + i));
> >>> +}
> >>> +
> >>>  /*
> >>>   * Called after dropping swapcache to decrease refcnt to swap entries.
> >>>   */
> >>
> >> Thanks,
> >> Ryan
> >>
> >>
> >
> >
>


-- 
Thanks,
Chuanhua