Messages in this thread | | | Subject | Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers | From | Christophe Leroy <> | Date | Sat, 7 Mar 2020 08:05:09 +0100 |
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Le 07/03/2020 à 01:56, Anshuman Khandual a écrit : > > > On 03/07/2020 06:04 AM, Qian Cai wrote: >> >> >>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote: >>> >>> Hmm, set_pte_at() function is not preferred here for these tests. The idea >>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >>> of 'static' tests. set_pte_at() is platform provided and could/might trigger >>> these flushes or some other platform specific synchronization stuff. Just >> >> Why is that important for this debugging option? > > Primarily reason is to avoid TLB/cache flush instructions on the system > during these tests that only involve transforming different page table > level entries through helpers. Unless really necessary, why should it > emit any TLB/cache flush instructions ?
What's the problem with thoses flushes ?
> >> >>> wondering is there specific reason with respect to the soft lock up problem >>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? >> >> Looks at the s390 version of set_pte_at(), it has this comment, >> vmaddr); >> >> /* >> * Certain architectures need to do special things when PTEs >> * within a page table are directly modified. Thus, the following >> * hook is made available. >> */ >> >> I can only guess that powerpc could be the same here. > > This comment is present in multiple platforms while defining set_pte_at(). > Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() > does as compared to WRITE_ONCE() that avoids the soft lock up, just trying > to understand. >
Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess.
To properly set the page table entries, set_pte_at() has to be used: - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't. - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't.
set_pte_at() also does a few other mandatory things, like calling pte_mkpte()
So, the WRITE_ONCE() must definitely become a set_pte_at()
Christophe
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