[PATCH 3/4] mm: shmem: don't truncate page if memory failure happens

[Yang Shi]

The current behavior of memory failure is to truncate the page cache
regardless of dirty or clean.  If the page is dirty the later access
will get the obsolete data from disk without any notification to the
users.  This may cause silent data loss.  It is even worse for shmem
since shmem is in-memory filesystem, truncating page cache means
discarding data blocks.  The later read would return all zero.

The right approach is to keep the corrupted page in page cache, any
later access would return error for syscalls or SIGBUS for page fault,
until the file is truncated, hole punched or removed.  The regular
storage backed filesystems would be more complicated so this patch
is focused on shmem.  This also unblock the support for soft
offlining shmem THP.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/memory-failure.c |  3 ++-
 mm/shmem.c          | 25 +++++++++++++++++++++++--
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 54879c339024..3e06cb9d5121 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1101,7 +1101,8 @@ static int page_action(struct page_state *ps, struct page *p,
 	result = ps->action(p, pfn);
 
 	count = page_count(p) - 1;
-	if (ps->action == me_swapcache_dirty && result == MF_DELAYED)
+	if ((ps->action == me_swapcache_dirty && result == MF_DELAYED) ||
+	    (ps->action == me_pagecache_dirty && result == MF_FAILED))
 		count--;
 	if (count > 0) {
 		pr_err("Memory failure: %#lx: %s still referenced by %d users\n",
diff --git a/mm/shmem.c b/mm/shmem.c
index 88742953532c..ec33f4f7173d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2456,6 +2456,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 	struct inode *inode = mapping->host;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	pgoff_t index = pos >> PAGE_SHIFT;
+	int ret = 0;
 
 	/* i_rwsem is held by caller */
 	if (unlikely(info->seals & (F_SEAL_GROW |
@@ -2466,7 +2467,19 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
 			return -EPERM;
 	}
 
-	return shmem_getpage(inode, index, pagep, SGP_WRITE);
+	ret = shmem_getpage(inode, index, pagep, SGP_WRITE);
+
+	if (!ret) {
+		if (*pagep) {
+			if (PageHWPoison(*pagep)) {
+				unlock_page(*pagep);
+				put_page(*pagep);
+				ret = -EIO;
+			}
+		}
+	}
+
+	return ret;
 }
 
 static int
@@ -2555,6 +2568,11 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 			unlock_page(page);
 		}
 
+		if (page && PageHWPoison(page)) {
+			error = -EIO;
+			break;
+		}
+
 		/*
 		 * We must evaluate after, since reads (unlike writes)
 		 * are called without i_rwsem protection against truncate
@@ -3782,7 +3800,6 @@ const struct address_space_operations shmem_aops = {
 #ifdef CONFIG_MIGRATION
 	.migratepage	= migrate_page,
 #endif
-	.error_remove_page = generic_error_remove_page,
 };
 EXPORT_SYMBOL(shmem_aops);
 
@@ -4193,6 +4210,10 @@ struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
 		page = ERR_PTR(error);
 	else
 		unlock_page(page);
+
+	if (PageHWPoison(page))
+		page = NULL;
+
 	return page;
 #else
 	/*
-- 
2.26.2