Question on pte bits

Question on pte bits

[Martin Maletinsky]

Hello,

Looking at some memory managment functions in 2.4.18, I came accross the function follow_page, which is called by get_user_pages, to parse a processes page tables.

After getting the corresponding page table entry, the function makes a check, which I don't quite understand - if write access is requested to the page, it not only checks
the write permission in the page table entry (with pte_write()), but also the dirty bit (with pte_dirty()). Why does a page need to be dirty in the case write == 1 (see
line 444 in the code excerpt below?

Thanks in advance for any help
with best regards
Martin Maletinsky

P.S. Please put me on CC: in your reply, since I am not in the mailing list.

static struct page * follow_page(struct mm_struct *mm, unsigned long address, int write) 
425 {
426         pgd_t *pgd;
427         pmd_t *pmd;
428         pte_t *ptep, pte;
429 
430         pgd = pgd_offset(mm, address);
431         if (pgd_none(*pgd) || pgd_bad(*pgd))
432                 goto out;
433 
434         pmd = pmd_offset(pgd, address);
435         if (pmd_none(*pmd) || pmd_bad(*pmd))
436                 goto out;
437 
438         ptep = pte_offset_atomic(pmd, address);
439 
440         pte = *ptep;
441         pte_kunmap(ptep);
442         if (pte_present(pte)) {
443                 if (!write ||
444                     (pte_write(pte) && pte_dirty(pte)))
445                         return pte_page(pte);
446         }
447 
448 out:
449         return 0;
450 }


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Re: Question on pte bits

[Rik van Riel]

On Fri, 6 Dec 2002, Martin Maletinsky wrote:

> After getting the corresponding page table entry, the function makes a
> check, which I don't quite understand - if write access is requested to
> the page, it not only checks the write permission in the page table
> entry (with pte_write()), but also the dirty bit (with pte_dirty()). Why
> does a page need to be dirty in the case write == 1 (see line 444 in the
> code excerpt below?

If write == 1, then somebody wants to write to the page NOW.
In that case it's more efficient to just set the dirty bit
than to take a trap later on; remember that many CPUs can't
keep track of the dirty bit in hardware but trap to the OS.

Rik
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