linux-kernel - Re: [PATCH v2 07/12] s390: add pte_free

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Message-ID: <c8aeb847-3d91-b5e5-48d1-20d411b58574@google.com>
Date:   Tue, 4 Jul 2023 09:03:29 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
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Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing
 page

On Tue, 4 Jul 2023, Alexander Gordeev wrote:
> On Sat, Jul 01, 2023 at 09:32:38PM -0700, Hugh Dickins wrote:
> > On Thu, 29 Jun 2023, Hugh Dickins wrote:
> 
> Hi Hugh,
> 
> ...
> > No, not quite the same rules as before: I came to realize that using
> > list_add_tail() for the HH pages would be liable to put a page on the
> > list which forever blocked reuse of PP list_add_tail() pages after it
> > (could be solved by a list_move() somewhere, but we have agreed to
> > prefer simplicity).
> 
> Just to make things more clear for me: do I understand correctly that this
> was an attempt to add HH fragments to pgtable_list from pte_free_defer()?

Yes, from page_table_free() called from pte_free_defer(): I had claimed
they could be put on the list (or not) without needing to consider their
HH-ness, apart from wanting to list_add_tail() rather than list_add() them.

But then realized that this category of list_add_tail() pages would block
access to the others.

But I think I was mistaken then to say "could be solved by a list_move()
somewhere"; because "somewhere" would have had to be __tlb_remove_table()
when it removes PP-bits, which would bring us back to the issues of
getting a spinlock from an mm which might already be freed.

Hugh