Re: File THP and HWPoison

On Tue, Mar 16, 2021 at 09:36:49AM -0700, Andi Kleen wrote:

quoted

So we now map the PMD-sized page into userspace, even though it has a
HWPoison in it.

Yes that's bad. It might panic the system.

quoted

I think there are two things that we should be doing:

1. Attempt to split THPs which are file-backed.  That makes most of this
problem disappear because there won't be THPs with HWPoison, mostly.

That seems simple and sensible.

quoted

2. When the THP fails to split, use a spare page flag to indicate that
the THP contains a HWPoison bit in one of its subpages.  There are a
lot of PF_SECOND flags available for this purpose.

Why should it fail? I thought splitting always succeeds.

If somebody has a temporary reference to it, splitting will fail.
eg a process is currently doing a read() from it, so filemap_read()
has a reference on the page.  This is why THPs support a delayed split
(currently used under memory pressure, but we could do something using
that delayed list to schedule the page for splitting later if it has
HWPoison).