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pascal@12457
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1 --- linux-2.6.37/include/linux/mm.h
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2 +++ linux-2.6.37/include/linux/mm.h
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3 @@ -1415,6 +1415,7 @@
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4 #define FOLL_GET 0x04 /* do get_page on page */
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5 #define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
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6 #define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
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7 +#define FOLL_COW 0x4000 /* internal GUP flag */
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8
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9 typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
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10 void *data);
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11 --- linux-2.6.37/mm/memory.c
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12 +++ linux-2.6.37/mm/memory.c
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13 @@ -1225,6 +1225,24 @@
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14 }
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15 EXPORT_SYMBOL_GPL(zap_vma_ptes);
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16
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17 +static inline bool can_follow_write_pte(pte_t pte, struct page *page,
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18 + unsigned int flags)
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19 +{
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20 + if (pte_write(pte))
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21 + return true;
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22 +
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23 + /*
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24 + * Make sure that we are really following CoWed page. We do not really
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25 + * have to care about exclusiveness of the page because we only want
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26 + * to ensure that once COWed page hasn't disappeared in the meantime
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27 + * or it hasn't been merged to a KSM page.
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28 + */
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29 + if ((flags & FOLL_FORCE) && (flags & FOLL_COW))
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30 + return page && PageAnon(page) && !PageKsm(page);
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31 +
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32 + return false;
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33 +}
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34 +
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35 /**
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36 * follow_page - look up a page descriptor from a user-virtual address
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37 * @vma: vm_area_struct mapping @address
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38 @@ -1286,10 +1304,13 @@
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39 pte = *ptep;
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40 if (!pte_present(pte))
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41 goto no_page;
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42 - if ((flags & FOLL_WRITE) && !pte_write(pte))
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43 - goto unlock;
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44
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45 page = vm_normal_page(vma, address, pte);
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46 + if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, page, flags)) {
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47 + pte_unmap_unlock(ptep, ptl);
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48 + return NULL;
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49 + }
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50 +
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51 if (unlikely(!page)) {
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52 if ((flags & FOLL_DUMP) ||
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53 !is_zero_pfn(pte_pfn(pte)))
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54 @@ -1310,7 +1331,7 @@
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55 */
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56 mark_page_accessed(page);
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57 }
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58 -unlock:
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59 +
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60 pte_unmap_unlock(ptep, ptl);
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61 out:
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62 return page;
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63 @@ -1464,17 +1485,13 @@
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64 * The VM_FAULT_WRITE bit tells us that
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65 * do_wp_page has broken COW when necessary,
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66 * even if maybe_mkwrite decided not to set
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67 - * pte_write. We can thus safely do subsequent
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68 - * page lookups as if they were reads. But only
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69 - * do so when looping for pte_write is futile:
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70 - * in some cases userspace may also be wanting
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71 - * to write to the gotten user page, which a
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72 - * read fault here might prevent (a readonly
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73 - * page might get reCOWed by userspace write).
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74 + * pte_write. We cannot simply drop FOLL_WRITE
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75 + * here because the COWed page might be gone by
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76 + * the time we do the subsequent page lookups.
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77 */
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78 if ((ret & VM_FAULT_WRITE) &&
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79 !(vma->vm_flags & VM_WRITE))
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80 - foll_flags &= ~FOLL_WRITE;
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81 + foll_flags |= FOLL_COW;
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82
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83 cond_resched();
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84 }
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