Hi Alexey,
I was finally forced to revisit this for my glibc tst-tls3-malloc deadlock. And
indeed with this change we don'tsee the deadlock. But see below..
> @@ -139,12 +139,16 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
> */
> fault = handle_mm_fault(vma, address, flags);
>
> - /* If Pagefault was interrupted by SIGKILL, exit page fault "early" */
> + /* If we need to retry but a fatal signal is pending, handle the
> + * signal first. We do not need to release the mmap_sem because
> + * it would already be released in __lock_page_or_retry in
> + * mm/filemap.c. */
Right and we were already doing that: up_read() was called for !VM_FAULT_RETRY
meaning we relied on the core mm to do that already for VM_FAULT_RETRY case.
The issue here was additional check for VM_FAULT_ERROR. Typically this is not set
by handle_mm_fault() meaning for common user faults with signal pending, we were
not calling up_read, hence the ensuing deadlock.
> if (unlikely(fatal_signal_pending(current))) {
> - if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY))
> - up_read(&mm->mmap_sem);
> - if (user_mode(regs))
> + if (fault & VM_FAULT_RETRY) {
> + if (!user_mode(regs))
> + goto no_context;
Given this code is really tricky, lets only solve one problem with 1 one patch.
> return;
> + }
> }
The fault handling is spaghetti mess of checks and more checks and has not really
been touched since upstreaming. I need to clean it up and essentially rewrite it
for v4.19
