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SubjectRe: gdb strangness Under 2.3.11-pre1


On Sun, 18 Jul 1999, Tim Waugh wrote:
>
> No, Linus' fault. ;-)

Oops, indeed.

Thanks, patch obviously correct and applied.

However, I further _suspect_ that the test should be extended to do
"!pte_dirty()" too, but I wouldn't want to add that without testing. As
you have an obvious test-case, would you mind trying with the following
modified version:

> + if (write && (!pte_write(*pgtable) || !pte_dirty(*pgtable)))
> + goto fault_in_page;

which also makes sure that the page will always be marked dirty after the
thing.

> We probably have to mark the page dirty if (write) as well; exercise for
> the reader I guess. The 'if (pgd_none(*pgdir))' looks suspicious too --
> should that that be !pgd_present?

(pgd_none() <=> !pgd_present) for all cases except for the error cases.
Linux does not support swapped out page tables - it adds a surprising
amount to complexity for a fairly small gain.

So the stickler version to test a page directory entry is:

if (pgd_none(entry)) goto no-entry
if (pgd_bad(entry)) goto bad-entry
.. it's present and usable ..
so pgd/pmd_present is basically not really ever usable (except as part of
the pgd/pmd_bad() test, where a non-present pmd/pgd is always considered
bad).

Linus


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