linux-kernel - Re: [PATCH v2 3/3] mm/madvise: free hwpoison and swapin error entry in madvise_free_pte

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Date:   Thu, 21 Apr 2022 22:52:59 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, neilb@...e.de, david@...hat.com,
        apopple@...dia.com, surenb@...gle.com, minchan@...nel.org,
        sfr@...b.auug.org.au, naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] mm/madvise: free hwpoison and swapin error entry
 in madvise_free_pte_range

On Fri, Apr 22, 2022 at 10:47:32AM +0800, Miaohe Lin wrote:
> On 2022/4/21 22:28, Peter Xu wrote:
> > On Thu, Apr 21, 2022 at 08:53:48PM +0800, Miaohe Lin wrote:
> >> Once the MADV_FREE operation has succeeded, callers can expect they might
> >> get zero-fill pages if accessing the memory again. Therefore it should be
> >> safe to delete the hwpoison entry and swapin error entry. There is no
> >> reason to kill the process if it has called MADV_FREE on the range.
> >>
> >> Suggested-by: Alistair Popple <apopple@...dia.com>
> >> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> >> ---
> >>  mm/madvise.c | 13 ++++++++-----
> >>  1 file changed, 8 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/mm/madvise.c b/mm/madvise.c
> >> index 4d6592488b51..5f4537511532 100644
> >> --- a/mm/madvise.c
> >> +++ b/mm/madvise.c
> >> @@ -624,11 +624,14 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> >>  			swp_entry_t entry;
> >>  
> >>  			entry = pte_to_swp_entry(ptent);
> >> -			if (non_swap_entry(entry))
> >> -				continue;
> >> -			nr_swap--;
> >> -			free_swap_and_cache(entry);
> >> -			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > 
> > Nitpick: IMHO you don't need to invert non_swap_entry() then it'll generate
> > a smaller diff, just add the new code above "continue".
> 
> I tried this way, but that lead to long line splitting, so I rewrote the code like this.
> If you prefer to just add the new code above "continue", I will do it in the next version.

No worry then, feel free to keep it as is.

> 
> > 
> >> +			if (!non_swap_entry(entry)) {
> >> +				nr_swap--;
> >> +				free_swap_and_cache(entry);
> >> +				pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> >> +			} else if (is_hwpoison_entry(entry) ||
> >> +				   is_swapin_error_entry(entry)) {
> >> +				pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> > 
> > Since it's been discussed and you're reposting a new version anyway, why
> > not start with either reusing hwpoison or pte markers?  Or do you think it
> > should be for future to drop the new swap entry again?
> > 
> 
> IMHO if reusing hwpoison markers, there are some places that we need to distinguish them and do
> different processing (and maybe also well comment them) which will make code more complicated and
> somewhat hard to follow. And the "swapin error marker" here is most straightforward. And If pte markers
> will support the "swapin error case" in the future, I think it's fine to change to use it then.
> Does this make sense for you?

Yeah it's fine.  If the pte marker things can finally land as expected,
maybe I can try it out as the 2nd user of it. :)

-- 
Peter Xu