[PATCH v12 23/31] mm: don't do swap readahead during speculative page fault

[Laurent Dufour]

Vinayak Menon faced a panic because one thread was page faulting a page in
swap, while another one was mprotecting a part of the VMA leading to a VMA
split.
This raise a panic in swap_vma_readahead() because the VMA's boundaries
were not more matching the faulting address.

To avoid this, if the page is not found in the swap, the speculative page
fault is aborted to retry a regular page fault.

Reported-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
---
 mm/memory.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 6e6bf61c0e5c..1991da97e2db 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2900,6 +2900,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 				lru_cache_add_anon(page);
 				swap_readpage(page, true);
 			}
+		} else if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
+			/*
+			 * Don't try readahead during a speculative page fault
+			 * as the VMA's boundaries may change in our back.
+			 * If the page is not in the swap cache and synchronous
+			 * read is disabled, fall back to the regular page
+			 * fault mechanism.
+			 */
+			delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
+			ret = VM_FAULT_RETRY;
+			goto out;
 		} else {
 			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
 						vmf);
-- 
2.21.0