linux-kernel - Re: [PATCH v2 31/32] mm/swap: swap_vma_readahead() do the pte_offset

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Date:   Tue, 13 Jun 2023 20:58:26 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Huang, Ying" <ying.huang@...el.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 31/32] mm/swap: swap_vma_readahead() do the
 pte_offset_map()

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@...gle.com> 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@...gle.com>
> > ---
> >  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@...el.com>

Great, thanks a lot.

Hugh