[PATCH 3/6] mm/memory-failure: keep the page online while handling it

["Gregory Price (Meta)" <gourry@gourry.net>]

memory_failure(), soft_offline_page() and unpoison_memory() check that a
pfn is online, then sleep on mf_mutex before using its page.  If the
block goes offline meanwhile, a DEBUG_VM kernel faults on the poisoned
memmap, and removal frees it on any kernel:

  CPU 1 (soft_offline_page)           CPU 2 (offline_pages)
  pfn_to_online_page()
    page is online
  mutex_lock(&mf_mutex)
                                      offline_mem_sections()
                                      remove_pfn_range_from_zone()
                                        page_init_poison()
  get_hwpoison_page()
    zone_pcp_disable(page_zone(page))
      zone of a poisoned page

  Oops: general protection fault, probably for non-canonical address 0xdffffc000000069e: 0000 [#1] SMP KASAN
  RIP: 0010:__zone_set_pageset_high_and_batch+0x89/0x270
   zone_pcp_disable+0x28/0x40
   get_hwpoison_page+0xc2/0x1160
   soft_offline_page.part.0+0xe7/0xbf0

memory_failure() faults the same way.  unpoison_memory() does not fault,
but it clears flags on the poisoned page and returns success.

Hold get_online_mems() from before the online check to the last use of
the page.  soft_offline_page() took it only around get_hwpoison_page();
it now takes it once, earlier.

With a kprobe parking each entry point across the offline, base faults,
or for unpoison_memory() succeeds, in 3 of 3 runs each.  With this patch
all 9 runs wait and fail with -EIO or -ENXIO, and lockdep stays clean
over offline/online cycles mixed with error injection.

Fixes: 96c804a6ae8c ("mm/memory-failure.c: don't access uninitialized memmaps in memory_failure()")
Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
Fixes: d613f53c83ec ("mm/memory-failure: fix VM_BUG_ON_PAGE(PagePoisoned(page)) when unpoison memory")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/memory-failure.c | 31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 0c96eb5119976..cdae2a1988bb8 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2394,6 +2394,8 @@ int memory_failure(unsigned long pfn, int flags)
 	if (!sysctl_memory_failure_recovery)
 		panic("Memory failure on page %lx", pfn);
 
+	/* Keep the page online from the check below until we are done */
+	get_online_mems();
 	mutex_lock(&mf_mutex);
 
 	if (!(flags & MF_SW_SIMULATED))
@@ -2608,11 +2610,13 @@ int memory_failure(unsigned long pfn, int flags)
 identify_page_state:
 	res = identify_page_state(pfn, p, page_flags);
 	mutex_unlock(&mf_mutex);
+	put_online_mems();
 	return res;
 unlock_page:
 	folio_unlock(folio);
 unlock_mutex:
 	mutex_unlock(&mf_mutex);
+	put_online_mems();
 	return res;
 }
 EXPORT_SYMBOL_GPL(memory_failure);
@@ -2742,9 +2746,12 @@ int unpoison_memory(unsigned long pfn)
 	static DEFINE_RATELIMIT_STATE(unpoison_rs, DEFAULT_RATELIMIT_INTERVAL,
 					DEFAULT_RATELIMIT_BURST);
 
+	get_online_mems();
 	p = pfn_to_online_page(pfn);
-	if (!p)
+	if (!p) {
+		put_online_mems();
 		return -EIO;
+	}
 	folio = page_folio(p);
 
 	mutex_lock(&mf_mutex);
@@ -2833,6 +2840,7 @@ int unpoison_memory(unsigned long pfn)
 		unpoison_pr_info("%#lx: software-unpoisoned page\n",
 				 page_to_pfn(p), &unpoison_rs);
 	}
+	put_online_mems();
 	return ret;
 }
 EXPORT_SYMBOL(unpoison_memory);
@@ -2974,38 +2982,42 @@ int soft_offline_page(unsigned long pfn, int flags)
 		return -ENXIO;
 	}
 
+	if (!sysctl_enable_soft_offline) {
+		pr_info_once("disabled by /proc/sys/vm/enable_soft_offline\n");
+		put_ref_page(pfn, flags);
+		return -EOPNOTSUPP;
+	}
+
+	/* Keep the page online from the check below until we are done */
+	get_online_mems();
+
 	/* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
 	page = pfn_to_online_page(pfn);
 	if (!page) {
+		put_online_mems();
 		put_ref_page(pfn, flags);
 		return -EIO;
 	}
 
-	if (!sysctl_enable_soft_offline) {
-		pr_info_once("disabled by /proc/sys/vm/enable_soft_offline\n");
-		put_ref_page(pfn, flags);
-		return -EOPNOTSUPP;
-	}
-
 	mutex_lock(&mf_mutex);
 
 	if (PageHWPoison(page)) {
 		pr_info("%#lx: page already poisoned\n", pfn);
 		put_ref_page(pfn, flags);
 		mutex_unlock(&mf_mutex);
+		put_online_mems();
 		return 0;
 	}
 
 retry:
-	get_online_mems();
 	ret = get_hwpoison_page(page, flags | MF_SOFT_OFFLINE);
-	put_online_mems();
 
 	if (hwpoison_filter(page)) {
 		if (ret > 0)
 			put_page(page);
 
 		mutex_unlock(&mf_mutex);
+		put_online_mems();
 		return -EOPNOTSUPP;
 	}
 
@@ -3023,6 +3035,7 @@ int soft_offline_page(unsigned long pfn, int flags)
 	}
 
 	mutex_unlock(&mf_mutex);
+	put_online_mems();
 
 	return ret;
 }
-- 
2.53.0-Meta