Re: [PATCH v2 31/32] mm/swap: swap_vma_readahead() do the pte_offset_map()

[Hugh Dickins]

On Mon, 12 Jun 2023, Huang, Ying wrote:

> Hi, Hugh,
> 
> Sorry for late reply.

Never apologize to *me* for being "late" or "slow" or "unresponsive".

Thanks for looking, yes, it was indeed for this one that I particularly
added you to the Cc.

> 
> Hugh Dickins <hughd@xxxxxxxxxx> writes:
> 
> > swap_vma_readahead() has been proceeding in an unconventional way, its
> > preliminary swap_ra_info() doing the pte_offset_map() and pte_unmap(),
> > then relying on that pte pointer even after the pte_unmap() - in its
> > CONFIG_64BIT case (I think !CONFIG_HIGHPTE was intended; whereas 32-bit
> > copied ptes to stack while they were mapped, but had to limit how many).
> >
> > Though it would be difficult to construct a failing testcase, accessing
> > page table after pte_unmap() will become bad practice, even on 64-bit:
> > an rcu_read_unlock() in pte_unmap() will allow page table to be freed.
> >
> > Move relevant definitions from include/linux/swap.h to mm/swap_state.c,
> > nothing else used them.  Delete the CONFIG_64BIT distinction and buffer,
> > delete all reference to ptes from swap_ra_info(), use pte_offset_map()
> > repeatedly in swap_vma_readahead(), breaking from the loop if it fails.
> >
> > (Will the repeated "map" and "unmap" show up as a slowdown anywhere?
> > If so, maybe modify __read_swap_cache_async() to do the pte_unmap()
> > only when it does not find the page already in the swapcache.)
> >
> > Use ptep_get_lockless(), mainly for its READ_ONCE().  Correctly advance
> > the address passed down to each call of __read__swap_cache_async().
> >
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > ---
> >  include/linux/swap.h | 19 -------------------
> >  mm/swap_state.c      | 45 +++++++++++++++++++++++---------------------
> >  2 files changed, 24 insertions(+), 40 deletions(-)

...

> Because we don't deal with PTEs in struct vma_swap_readahead anymore, it
> appears simpler to record addresses directly, for example,
> 
> struct vma_swap_readahead {
> 	unsigned long start;
> 	unsigned long end;
> };
> 
> we can make ra_info.win to be the return value of swap_ra_info().
> 
> Anyway, this can be a separate cleanup patch based on this patch.

Ooh, that would have required me to think, rather than just delete
lines.  Mmm, if you see a cleaner way forward, yes, please do add
some cleanup on top.

> 
> For the patch itself, feel free to add,
> 
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Great, thanks a lot.

Hugh