Hi Suren and Miaohe
Thank you all for taking the time to discuss this issue.
On 8/23/24 06:50, Suren Baghdasaryan wrote:
On Thu, Aug 22, 2024 at 2:46 AM Hao Ge <hao.ge@xxxxxxxxx> wrote:
Hi Miaohe
Thank you for taking the time to review this patch.
On 8/22/24 16:04, Miaohe Lin wrote:
On 2024/8/22 10:58, Hao Ge wrote:
From: Hao Ge <gehao@xxxxxxxxxx>
Thanks for your patch.
The PG_hwpoison page will be caught and isolated on the entrance to
the free buddy page pool. so,when we clear this flag and return it
to the buddy system,mark codetags for pages as empty.
Is below scene cause the problem?
1. Pages are allocated. pgalloc_tag_add() will be called when prep_new_page().
2. Pages are hwpoisoned. memory_failure() will set PG_hwpoison flag and pgalloc_tag_sub()
will be called when pages are caught and isolated on the entrance to buddy.
Hi Folks,
Thanks for reporting this! Could you please describe in more details
how memory_failure() ends up calling pgalloc_tag_sub()? It's not
obvious to me which path leads to pgalloc_tag_sub(), so I must be
missing something.
OK,Let me describe the scenario I encountered.
In the Link [1] I mentioned,here is the logic behind it:
It performed the following operations:
madvise(ptrs[num_alloc], pagesize, MADV_SOFT_OFFLINE)
and then the kernel's call stack looks like this:
do_madvise
soft_offline_page
page_handle_poison
__folio_put
free_unref_page
It will set a flag within the following function and then release the page.
https://elixir.bootlin.com/linux/v6.11-rc4/source/mm/memory-failure.c#L206
and and then,because you set the PG_hwpoison flag, so the page will be
caught and isolated on the
entrance to the free buddy page pool. look here:
https://elixir.bootlin.com/linux/v6.11-rc4/source/mm/page_alloc.c#L1052
At this very moment, we call pgalloc_tag_sub.
So,when we callunpoison_memoryclear this flag and return the page to the
buddy system, the problem arises.
On a conceptual level I want to understand if the page isolated in
this manner should be considered freed or not. If it shouldn't be
considered free then I think the right fix would be to avoid
pgalloc_tag_sub() when this isolation happens.
Thanks,
Suren.
In my understanding, the purpose of unpoison_memory is to reclaim
poisoned pages.
I dug up the patch that introduced this function back then
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/mm/memory-failure.c?id=847ce401df392b0704369fd3f75df614ac1414b4
Therefore, this is reasonable.
Thanks
Best regards
Hao
3. unpoison_memory cleared flags and sent the pages to buddy. pgalloc_tag_sub() will be
called again in free_pages_prepare().
So there is a imbalance that pgalloc_tag_add() is called once and pgalloc_tag_sub() is called twice?
As you said, that's exactly the case.
If so, let's think about more complicated scene:
1. Same as above.
2. Pages are hwpoisoned. But memory_failure() fails to handle it. So PG_hwpoison flag is set
but pgalloc_tag_sub() is not called (pages are not sent to buddy).
3. unpoison_memory cleared flags and calls clear_page_tag_ref() without calling pgalloc_tag_sub()
first. Will this cause problem?
Though this should be really rare...
Thanks.
.
Great, I didn't anticipate this scenario.
When we call clear_page_tag_ref() without calling pgalloc_tag_sub(),
It will cause exceptions in|tag->counters->bytes|and|tag->counters->calls|.
We can add a layer of protection to handle it
The pseudocode is as follows:
if (mem_alloc_profiling_enabled()) {
union codetag_ref *ref = get_page_tag_ref(page);
if (ref) {
if( ref->ct != NULL && !is_codetag_empty(ref))
{
tag = ct_to_alloc_tag(ref->ct);
this_cpu_sub(tag->counters->bytes, bytes);
this_cpu_dec(tag->counters->calls);
}
set_codetag_empty(ref);
put_page_tag_ref(ref);
}
}
Hi Suren and Kent
Do you have any suggestions for this? If it's okay, I'll add comments
and include this pseudocode in|clear_page_tag_ref|.
It was detected by [1] and the following WARN occurred:
[ 113.930443][ T3282] ------------[ cut here ]------------
[ 113.931105][ T3282] alloc_tag was not set
[ 113.931576][ T3282] WARNING: CPU: 2 PID: 3282 at ./include/linux/alloc_tag.h:130 pgalloc_tag_sub.part.66+0x154/0x164
[ 113.932866][ T3282] Modules linked in: hwpoison_inject fuse ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ebtable_nat ebtable_broute ip6table_nat ip6table_man4
[ 113.941638][ T3282] CPU: 2 UID: 0 PID: 3282 Comm: madvise11 Kdump: loaded Tainted: G W 6.11.0-rc4-dirty #18
[ 113.943003][ T3282] Tainted: [W]=WARN
[ 113.943453][ T3282] Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022
[ 113.944378][ T3282] pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 113.945319][ T3282] pc : pgalloc_tag_sub.part.66+0x154/0x164
[ 113.946016][ T3282] lr : pgalloc_tag_sub.part.66+0x154/0x164
[ 113.946706][ T3282] sp : ffff800087093a10
[ 113.947197][ T3282] x29: ffff800087093a10 x28: ffff0000d7a9d400 x27: ffff80008249f0a0
[ 113.948165][ T3282] x26: 0000000000000000 x25: ffff80008249f2b0 x24: 0000000000000000
[ 113.949134][ T3282] x23: 0000000000000001 x22: 0000000000000001 x21: 0000000000000000
[ 113.950597][ T3282] x20: ffff0000c08fcad8 x19: ffff80008251e000 x18: ffffffffffffffff
[ 113.952207][ T3282] x17: 0000000000000000 x16: 0000000000000000 x15: ffff800081746210
[ 113.953161][ T3282] x14: 0000000000000000 x13: 205d323832335420 x12: 5b5d353031313339
[ 113.954120][ T3282] x11: ffff800087093500 x10: 000000000000005d x9 : 00000000ffffffd0
[ 113.955078][ T3282] x8 : 7f7f7f7f7f7f7f7f x7 : ffff80008236ba90 x6 : c0000000ffff7fff
[ 113.956036][ T3282] x5 : ffff000b34bf4dc8 x4 : ffff8000820aba90 x3 : 0000000000000001
[ 113.956994][ T3282] x2 : ffff800ab320f000 x1 : 841d1e35ac932e00 x0 : 0000000000000000
[ 113.957962][ T3282] Call trace:
[ 113.958350][ T3282] pgalloc_tag_sub.part.66+0x154/0x164
[ 113.959000][ T3282] pgalloc_tag_sub+0x14/0x1c
[ 113.959539][ T3282] free_unref_page+0xf4/0x4b8
[ 113.960096][ T3282] __folio_put+0xd4/0x120
[ 113.960614][ T3282] folio_put+0x24/0x50
[ 113.961103][ T3282] unpoison_memory+0x4f0/0x5b0
[ 113.961678][ T3282] hwpoison_unpoison+0x30/0x48 [hwpoison_inject]
[ 113.962436][ T3282] simple_attr_write_xsigned.isra.34+0xec/0x1cc
[ 113.963183][ T3282] simple_attr_write+0x38/0x48
[ 113.963750][ T3282] debugfs_attr_write+0x54/0x80
[ 113.964330][ T3282] full_proxy_write+0x68/0x98
[ 113.964880][ T3282] vfs_write+0xdc/0x4d0
[ 113.965372][ T3282] ksys_write+0x78/0x100
[ 113.965875][ T3282] __arm64_sys_write+0x24/0x30
[ 113.966440][ T3282] invoke_syscall+0x7c/0x104
[ 113.966984][ T3282] el0_svc_common.constprop.1+0x88/0x104
[ 113.967652][ T3282] do_el0_svc+0x2c/0x38
[ 113.968893][ T3282] el0_svc+0x3c/0x1b8
[ 113.969379][ T3282] el0t_64_sync_handler+0x98/0xbc
[ 113.969980][ T3282] el0t_64_sync+0x19c/0x1a0
[ 113.970511][ T3282] ---[ end trace 0000000000000000 ]---
Link [1]: https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/madvise/madvise11.c
Fixes: a8fc28dad6d5 ("alloc_tag: introduce clear_page_tag_ref() helper function")
Cc: stable@xxxxxxxxxxxxxxx # v6.10
Signed-off-by: Hao Ge <gehao@xxxxxxxxxx>
---
mm/memory-failure.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 7066fc84f351..570388c41532 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2623,6 +2623,12 @@ int unpoison_memory(unsigned long pfn)
folio_put(folio);
if (TestClearPageHWPoison(p)) {
+ /* the PG_hwpoison page will be caught and isolated
+ * on the entrance to the free buddy page pool.
+ * so,when we clear this flag and return it to the buddy system,
+ * clear it's codetag
+ */
+ clear_page_tag_ref(p);
folio_put(folio);
ret = 0;
}
Thanks
BR
Hao
