Messages in this thread | | | Date | Thu, 19 Nov 2020 14:03:58 -0500 | From | Andrea Arcangeli <> | Subject | Re: [PATCH 0/1] x86: restore the write back cache of reserved RAM in iounmap() |
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Hello Christoph,
On Thu, Nov 19, 2020 at 06:02:06PM +0000, Christoph Hellwig wrote: > What is the callers? The whole SetPageReservered + ioremap* thing > you mention in the actual patch is completely bogus. I think we'll > need to reject that as well and fix the caller.
The actual caller is not so much the focus here: the point here is to be able to either handle the caller gracefully or to get a synchronous kernel crash in __free_pages.
Otherwise the problem induced by such a caller (no matter if right or wrong) becomes hardly debuggable.
The caller in question was the EFI_BOOT_SERVICE_DATA that is aliased on non RAM but then freed later by swapping RAM under it.
Of course the caller has already been changed to stick to write back and that specific caller is not a concern anymore. My concern is if we leave the callee (iounmap) as it is, what does guarantee us that we won't hit again in production a few years down the road?
When I first read the caller it felt nothing should have gone wrong, it looked ok even the version that would leave PCD leftovers bits in the direct map. So I didn't get why switching to write back would prevent the PCD leftovers until I looked at the callee (iounmap).
Thanks, Andrea
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