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Subject[PATCH -next v2] mm/swapfile: fix and annotate various data races
Date
swap_info_struct si.highest_bit, si.swap_map[offset] and si.flags could
be accessed concurrently separately as noticed by KCSAN,

=== si.highest_bit ===

write to 0xffff8d5abccdc4d4 of 4 bytes by task 5353 on cpu 24:
swap_range_alloc+0x81/0x130
swap_range_alloc at mm/swapfile.c:681
scan_swap_map_slots+0x371/0xb90
get_swap_pages+0x39d/0x5c0
get_swap_page+0xf2/0x524
add_to_swap+0xe4/0x1c0
shrink_page_list+0x1795/0x2870
shrink_inactive_list+0x316/0x880
shrink_lruvec+0x8dc/0x1380
shrink_node+0x317/0xd80
do_try_to_free_pages+0x1f7/0xa10
try_to_free_pages+0x26c/0x5e0
__alloc_pages_slowpath+0x458/0x1290

read to 0xffff8d5abccdc4d4 of 4 bytes by task 6672 on cpu 70:
scan_swap_map_slots+0x4a6/0xb90
scan_swap_map_slots at mm/swapfile.c:892
get_swap_pages+0x39d/0x5c0
get_swap_page+0xf2/0x524
add_to_swap+0xe4/0x1c0
shrink_page_list+0x1795/0x2870
shrink_inactive_list+0x316/0x880
shrink_lruvec+0x8dc/0x1380
shrink_node+0x317/0xd80
do_try_to_free_pages+0x1f7/0xa10
try_to_free_pages+0x26c/0x5e0
__alloc_pages_slowpath+0x458/0x1290

Reported by Kernel Concurrency Sanitizer on:
CPU: 70 PID: 6672 Comm: oom01 Tainted: G W L 5.5.0-next-20200205+ #3
Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019

=== si.swap_map[offset] ===

write to 0xffffbc370c29a64c of 1 bytes by task 6856 on cpu 86:
__swap_entry_free_locked+0x8c/0x100
__swap_entry_free_locked at mm/swapfile.c:1209 (discriminator 4)
__swap_entry_free.constprop.20+0x69/0xb0
free_swap_and_cache+0x53/0xa0
unmap_page_range+0x7f8/0x1d70
unmap_single_vma+0xcd/0x170
unmap_vmas+0x18b/0x220
exit_mmap+0xee/0x220
mmput+0x10e/0x270
do_exit+0x59b/0xf40
do_group_exit+0x8b/0x180

read to 0xffffbc370c29a64c of 1 bytes by task 6855 on cpu 20:
_swap_info_get+0x81/0xa0
_swap_info_get at mm/swapfile.c:1140
free_swap_and_cache+0x40/0xa0
unmap_page_range+0x7f8/0x1d70
unmap_single_vma+0xcd/0x170
unmap_vmas+0x18b/0x220
exit_mmap+0xee/0x220
mmput+0x10e/0x270
do_exit+0x59b/0xf40
do_group_exit+0x8b/0x180

=== si.flags ===

write to 0xffff956c8fc6c400 of 8 bytes by task 6087 on cpu 23:
scan_swap_map_slots+0x6fe/0xb50
scan_swap_map_slots at mm/swapfile.c:887
get_swap_pages+0x39d/0x5c0
get_swap_page+0x377/0x524
add_to_swap+0xe4/0x1c0
shrink_page_list+0x1795/0x2870
shrink_inactive_list+0x316/0x880
shrink_lruvec+0x8dc/0x1380
shrink_node+0x317/0xd80
do_try_to_free_pages+0x1f7/0xa10
try_to_free_pages+0x26c/0x5e0
__alloc_pages_slowpath+0x458/0x1290

read to 0xffff956c8fc6c400 of 8 bytes by task 6207 on cpu 63:
_swap_info_get+0x41/0xa0
__swap_info_get at mm/swapfile.c:1114
put_swap_page+0x84/0x490
__remove_mapping+0x384/0x5f0
shrink_page_list+0xff1/0x2870
shrink_inactive_list+0x316/0x880
shrink_lruvec+0x8dc/0x1380
shrink_node+0x317/0xd80
do_try_to_free_pages+0x1f7/0xa10
try_to_free_pages+0x26c/0x5e0
__alloc_pages_slowpath+0x458/0x1290

The writes are under si->lock but the reads are not. For si.highest_bit
and si.swap_map[offset], data race could trigger logic bugs, so fix them
by having WRITE_ONCE() for the writes and READ_ONCE() for the reads
except those isolated reads where they compare against zero which a data
race would cause no harm. Thus, annotate them as intentional data races
using the data_race() macro.

For si.flags, the readers are only interested in a single bit where a
data race there would cause no issue there.

Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: add a missing annotation for si->flags in memory.c.

mm/memory.c | 4 ++--
mm/swapfile.c | 31 ++++++++++++++++++-------------
2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0bccc622e482..c972c10d6f39 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2955,8 +2955,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
if (!page) {
struct swap_info_struct *si = swp_swap_info(entry);

- if (si->flags & SWP_SYNCHRONOUS_IO &&
- __swap_count(entry) == 1) {
+ if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
+ __swap_count(entry) == 1) {
/* skip swapcache */
page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma,
vmf->address);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2c33ff456ed5..8178db2b9873 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -678,7 +678,7 @@ static void swap_range_alloc(struct swap_info_struct *si, unsigned long offset,
if (offset == si->lowest_bit)
si->lowest_bit += nr_entries;
if (end == si->highest_bit)
- si->highest_bit -= nr_entries;
+ WRITE_ONCE(si->highest_bit, si->highest_bit - nr_entries);
si->inuse_pages += nr_entries;
if (si->inuse_pages == si->pages) {
si->lowest_bit = si->max;
@@ -710,7 +710,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
if (end > si->highest_bit) {
bool was_full = !si->highest_bit;

- si->highest_bit = end;
+ WRITE_ONCE(si->highest_bit, end);
if (was_full && (si->flags & SWP_WRITEOK))
add_to_avail_list(si);
}
@@ -843,7 +843,7 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
else
goto done;
}
- si->swap_map[offset] = usage;
+ WRITE_ONCE(si->swap_map[offset], usage);
inc_cluster_info_page(si, si->cluster_info, offset);
unlock_cluster(ci);

@@ -889,12 +889,13 @@ static int scan_swap_map_slots(struct swap_info_struct *si,

scan:
spin_unlock(&si->lock);
- while (++offset <= si->highest_bit) {
- if (!si->swap_map[offset]) {
+ while (++offset <= READ_ONCE(si->highest_bit)) {
+ if (data_race(!si->swap_map[offset])) {
spin_lock(&si->lock);
goto checks;
}
- if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+ if (vm_swap_full() &&
+ READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
spin_lock(&si->lock);
goto checks;
}
@@ -905,11 +906,12 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
}
offset = si->lowest_bit;
while (offset < scan_base) {
- if (!si->swap_map[offset]) {
+ if (data_race(!si->swap_map[offset])) {
spin_lock(&si->lock);
goto checks;
}
- if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+ if (vm_swap_full() &&
+ READ_ONCE(si->swap_map[offset]) == SWAP_HAS_CACHE) {
spin_lock(&si->lock);
goto checks;
}
@@ -1111,7 +1113,7 @@ static struct swap_info_struct *__swap_info_get(swp_entry_t entry)
p = swp_swap_info(entry);
if (!p)
goto bad_nofile;
- if (!(p->flags & SWP_USED))
+ if (data_race(!(p->flags & SWP_USED)))
goto bad_device;
offset = swp_offset(entry);
if (offset >= p->max)
@@ -1137,7 +1139,7 @@ static struct swap_info_struct *_swap_info_get(swp_entry_t entry)
p = __swap_info_get(entry);
if (!p)
goto out;
- if (!p->swap_map[swp_offset(entry)])
+ if (data_race(!p->swap_map[swp_offset(entry)]))
goto bad_free;
return p;

@@ -1206,7 +1208,10 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
}

usage = count | has_cache;
- p->swap_map[offset] = usage ? : SWAP_HAS_CACHE;
+ if (usage)
+ WRITE_ONCE(p->swap_map[offset], usage);
+ else
+ WRITE_ONCE(p->swap_map[offset], SWAP_HAS_CACHE);

return usage;
}
@@ -1258,7 +1263,7 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
goto bad_nofile;

rcu_read_lock();
- if (!(si->flags & SWP_VALID))
+ if (data_race(!(si->flags & SWP_VALID)))
goto unlock_out;
offset = swp_offset(entry);
if (offset >= si->max)
@@ -3436,7 +3441,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
} else
err = -ENOENT; /* unused swap entry */

- p->swap_map[offset] = count | has_cache;
+ WRITE_ONCE(p->swap_map[offset], count | has_cache);

unlock_out:
unlock_cluster_or_swap_info(p, ci);
--
1.8.3.1
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