On 26.02.25 17:28, Matthew Wilcox wrote:
On Wed, Feb 26, 2025 at 04:42:46PM +0100, David Hildenbrand wrote:
On 26.02.25 15:03, Matthew Wilcox wrote:
On Wed, Feb 26, 2025 at 06:48:15AM -0500, Brian Geffon wrote:
When handling faults for anon shmem finish_fault() will attempt to install
ptes for the entire folio. Unfortunately if it encounters a single
non-pte_none entry in that range it will bail, even if the pte that
triggered the fault is still pte_none. When this situation happens the
fault will be retried endlessly never making forward progress.
This patch fixes this behavior and if it detects that a pte in the range
is not pte_none it will fall back to setting just the pte for the
address that triggered the fault.
Surely there's a similar problem in do_anonymous_page()?
I recall we handle it in there correctly the last time I stared at it.
We check pte_none to decide which folio size we can allocate (including
basing the decision on other factors like VMA etc), and after retaking the
PTL, we recheck vmf_pte_changed / pte_range_none() to make sure there were
no races.
Ah, so then we'll retry and allocate a folio of the right size the next
time?
IIRC we'll retry the fault in case we had a race. Likely, if we had a
race, somebody else installed a (large) folio and we essentially have to
second fault. If, for some reason, the race only touched parts of the
PTEs we tried to modify, we'll get another fault and allocate something
(smaller) that would fit into the new empty range.
So yes, we're more flexible because we're allocating the folios and
don't have to take whatever folio size is in the pagecache in consideration.
--
Cheers,
David / dhildenb
