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 }
--
Supercomputing System AG email: maletinsky@scs.ch
Martin Maletinsky phone: +41 (0)1 445 16 05
Technoparkstrasse 1 fax: +41 (0)1 445 16 10
CH-8005 Zurich
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
FAQ: http://kernelnewbies.org/faq/
