linux-kernel - Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap

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Date: Tue, 11 Jun 2024 11:53:11 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org, hannes@...xchg.org, 
	david@...hat.com, ying.huang@...el.com, hughd@...gle.com, willy@...radead.org, 
	chengming.zhou@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH v3 1/2] mm: store zero pages to be swapped out in a bitmap

On Tue, Jun 11, 2024 at 11:47 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> [..]
> > > @@ -1336,6 +1347,7 @@ 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);
> >
> > Hmm so clear_bit() is done at the swap_entry_free() point. I wonder if
> > we can have a problem, where:
> >
> > 1. The swap entry has its zeromap bit set, and is freed to the swap
> > slot cache (free_swap_slot() in mm/swap_slots.c). For instance, it is
> > reclaimed from the swap cache, and all the processes referring to it
> > are terminated, which decrements the swap count to 0 (swap_free() ->
> > __swap_entry_free() -> free_swap_slots())
> >
> > 2. The swap slot is then re-used in swap space allocation
> > (add_to_swap()) - its zeromap bit is never cleared.
>
> I do not think this can happen before swap_entry_free() is called.
> Note that when a swap entry is freed to the swap slot cache in
> free_swap_slot(), it is added to cache->slots_ret, not cache->slots.
> The former are swap entries cached to be later freed using
> swap_entry_free().

Ahhh I see. Good point. Then yeah this should be safe from this POV.

>
> >
> > 3. swap_writepage() writes that non-zero page to swap
> >
> > 4. swap_read_folio() checks the bitmap, sees that the zeromap bit for
> > the entry is set, so populates a zero page for it.
> >
> > zswap in the past has to carefully invalidate these leftover entries
> > quite carefully. Chengming then move the invalidation point to
> > free_swap_slot(), massively simplifying the logic.
>
> I think the main benefit of moving the invalidation point was avoiding
> leaving the compressed page in zswap until swap_entry_free() is
> called, which will happen only when the swap slot caches are drained.
>

This is true. In this case yeah there's probably not much difference
between clearing the bit here vs in swap_entry_free().

> >
> > I wonder if we need to do the same here?