Re: [PATCH v3 2/2] x86/sgx: Resolve EREMOVE page vs EAUG page data race

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On Tue, 28 May 2024 11:23:13 -0500, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

On 5/17/24 04:06, Dmitrii Kuvaiskii wrote:
...

First, why is SGX so special here?  How is the SGX problem different
than what the core mm code does?

--- a/arch/x86/kernel/cpu/sgx/encl.h
+++ b/arch/x86/kernel/cpu/sgx/encl.h
@@ -25,6 +25,9 @@
 /* 'desc' bit marking that the page is being reclaimed. */
 #define SGX_ENCL_PAGE_BEING_RECLAIMED	BIT(3)

+/* 'desc' bit marking that the page is being removed. */
+#define SGX_ENCL_PAGE_BEING_REMOVED	BIT(2)

Second, convince me that this _needs_ a new bit.  Why can't we just have
a bit that effectively means "return EBUSY if you see this bit when
handling a fault".


IIUC, reclaimer_writing_to_pcmd() also uses SGX_ENCL_PAGE_BEING_RECLAIMED to check if a page is about being reclaimed in order to prevent its VA slot fro being freed. So I think we do need separate bit for EREMOVE which does not write to VA slot?

BR
Haitao




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