Re: [RFC] Page table sharing

[Hugh Dickins <hugh@veritas.com>]

On Mon, 18 Feb 2002, Daniel Phillips wrote:
> On February 18, 2002 09:09 am, Hugh Dickins wrote:
> 
> > So how is the page_table_lock taken by swap_out effective when it's
> > dealing with a page table shared by another mm than the one it is
> > locking?  And when handling a read-fault, again no such code (but
> > when handling a write-fault, __pte_alloc has unshared in advance).
> >
> > Since copy_page_range would not copy shared page tables, I'm wrong to
> > point there.  But __pte_alloc does copy shared page tables (to unshare
> > them), and needs them to be stable while it does so: so locking against
> > swap_out really is required.  It also needs locking against read faults,
> > and they against each other: but there I imagine it's just a matter of
> > dropping the write arg to __pte_alloc, going back to pte_alloc again.
> 
> You're right about the read faults, wrong about swap_out.  In general you've 
> been more right than wrong, so thanks.  I'll post a new patch pretty soon and 
> I'd appreciate your comments.

On the read faults: I see no change there in the patch you then posted,
handle_mm_fault still calls __pte_alloc with write_access argument, so
concurrent read faults on the same pte can still slot the page into the
shared page table at the same time, doubly counting it - no problem if
it's the Reserved empty_zero_page, and I think no problem at present
if it's a SwapCache page, since that is PageLocked in the current tree
(but not in -aa, and in due course we should go Andrea's way there);
but if it's a file page the double count will leave it unfreeable.

On swap_out versus __pte_alloc: I was misreading it and you're almost
right there: but you do need to change that "pte_t pte = *src_ptb;"
to something atomic - hmm, do we have any primitive for doing that?
neither set_pte nor ptep_get_and_clear is right.  Otherwise, on PAE
HIGHMEM64G systems the two halves of "pte" could be assigned before
and after try_to_swap_out's ptep_get_and_clear.  But once you've got
"pte", yes, you're basing all your decisions on your one local copy,
that gives all the stability you need.

Hugh

(By the way, the patch you appended to your next mail did not apply
- I think you'd hand-edited an incidental irrelevant cleanup out of
the patch to memory.c, without adjusting its line counts; and also
had to edit "public_html/" out of the URL you gave.)

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