Change mincore() to count "mapped" pages rather than "cached" pages

The semantics of what "in core" means for the mincore() system call are
somewhat unclear, but Linux has always (since 2.3.52, which is when
mincore() was initially done) treated it as "page is available in page
cache" rather than "page is mapped in the mapping".

The problem with that traditional semantic is that it exposes a lot of
system cache state that it really probably shouldn't, and that users
shouldn't really even care about.

So let's try to avoid that information leak by simply changing the
semantics to be that mincore() counts actual mapped pages, not pages
that might be cheaply mapped if they were faulted (note the "might be"
part of the old semantics: being in the cache doesn't actually guarantee
that you can access them without IO anyway, since things like network
filesystems may have to revalidate the cache before use).

In many ways the old semantics were somewhat insane even aside from the
information leak issue. From the very beginning (and that beginning is
a long time ago: 2.3.52 was released in March 2000, I think), the code
had a comment saying

Later we can get more picky about what "in core" means precisely.

and this is that "later". Admittedly it is much later than is really
comfortable.

NOTE! This is a real semantic change, and it is for example known to
change the output of "fincore", since that program literally does a
mmmap without populating it, and then doing "mincore()" on that mapping
that doesn't actually have any pages in it.

I'm hoping that nobody actually has any workflow that cares, and the
info leak is real.

We may have to do something different if it turns out that people have
valid reasons to want the old semantics, and if we can limit the
information leak sanely.

Cc: Kevin Easton <kevin@guarana.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Masatake YAMATO <yamato@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Linus Torvalds 7 years ago 574823bf 94bd8a05

+13 -81

1 changed file

expand all

mincore.c

+13 -81

mm/mincore.c

··· 42 42 return 0; 43 43 } 44 44 45 - /* 46 - * Later we can get more picky about what "in core" means precisely. 47 - * For now, simply check to see if the page is in the page cache, 48 - * and is up to date; i.e. that no page-in operation would be required 49 - * at this time if an application were to map and access this page. 50 - */ 51 - static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff) 52 - { 53 - unsigned char present = 0; 54 - struct page *page; 55 - 56 - /* 57 - * When tmpfs swaps out a page from a file, any process mapping that 58 - * file will not get a swp_entry_t in its pte, but rather it is like 59 - * any other file mapping (ie. marked !present and faulted in with 60 - * tmpfs's .fault). So swapped out tmpfs mappings are tested here. 61 - */ 62 - #ifdef CONFIG_SWAP 63 - if (shmem_mapping(mapping)) { 64 - page = find_get_entry(mapping, pgoff); 65 - /* 66 - * shmem/tmpfs may return swap: account for swapcache 67 - * page too. 68 - */ 69 - if (xa_is_value(page)) { 70 - swp_entry_t swp = radix_to_swp_entry(page); 71 - page = find_get_page(swap_address_space(swp), 72 - swp_offset(swp)); 73 - } 74 - } else 75 - page = find_get_page(mapping, pgoff); 76 - #else 77 - page = find_get_page(mapping, pgoff); 78 - #endif 79 - if (page) { 80 - present = PageUptodate(page); 81 - put_page(page); 82 - } 83 - 84 - return present; 85 - } 86 - 87 - static int __mincore_unmapped_range(unsigned long addr, unsigned long end, 88 - struct vm_area_struct *vma, unsigned char *vec) 89 - { 90 - unsigned long nr = (end - addr) >> PAGE_SHIFT; 91 - int i; 92 - 93 - if (vma->vm_file) { 94 - pgoff_t pgoff; 95 - 96 - pgoff = linear_page_index(vma, addr); 97 - for (i = 0; i < nr; i++, pgoff++) 98 - vec[i] = mincore_page(vma->vm_file->f_mapping, pgoff); 99 - } else { 100 - for (i = 0; i < nr; i++) 101 - vec[i] = 0; 102 - } 103 - return nr; 104 - } 105 - 106 45 static int mincore_unmapped_range(unsigned long addr, unsigned long end, 107 46 struct mm_walk *walk) 108 47 { 109 - walk->private += __mincore_unmapped_range(addr, end, 110 - walk->vma, walk->private); 48 + unsigned char *vec = walk->private; 49 + unsigned long nr = (end - addr) >> PAGE_SHIFT; 50 + 51 + memset(vec, 0, nr); 52 + walk->private += nr; 111 53 return 0; 112 54 } 113 55 ··· 69 127 goto out; 70 128 } 71 129 130 + /* We'll consider a THP page under construction to be there */ 72 131 if (pmd_trans_unstable(pmd)) { 73 - __mincore_unmapped_range(addr, end, vma, vec); 132 + memset(vec, 1, nr); 74 133 goto out; 75 134 } 76 135 ··· 80 137 pte_t pte = *ptep; 81 138 82 139 if (pte_none(pte)) 83 - __mincore_unmapped_range(addr, addr + PAGE_SIZE, 84 - vma, vec); 140 + *vec = 0; 85 141 else if (pte_present(pte)) 86 142 *vec = 1; 87 143 else { /* pte is a swap entry */ 88 144 swp_entry_t entry = pte_to_swp_entry(pte); 89 145 90 - if (non_swap_entry(entry)) { 91 - /* 92 - * migration or hwpoison entries are always 93 - * uptodate 94 - */ 95 - *vec = 1; 96 - } else { 97 - #ifdef CONFIG_SWAP 98 - *vec = mincore_page(swap_address_space(entry), 99 - swp_offset(entry)); 100 - #else 101 - WARN_ON(1); 102 - *vec = 1; 103 - #endif 104 - } 146 + /* 147 + * migration or hwpoison entries are always 148 + * uptodate 149 + */ 150 + *vec = !!non_swap_entry(entry); 105 151 } 106 152 vec++; 107 153 }

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