Re: [PATCH -next v8 4/4] arm64: add cow to machine check safe

[Tong Tiangen]

在 2023/4/12 0:45, Catalin Marinas 写道:
> On Mon, Dec 19, 2022 at 12:00:08PM +0000, Tong Tiangen wrote:
> > At present, Recover from poison consumption from copy-on-write has been
> > supported[1], arm64 should also support this mechanism.
> >
> > Add new helper copy_mc_page() which provide a page copy implementation with
> > machine check safe. At present, only used in cow. In the future, we can
> > expand more scenes. As long as the consequences of page copy failure are
> > not fatal(eg: only affect user process), we can use this helper.
> >
> > The copy_mc_page() in copy_page_mc.S is largely borrows from copy_page()
> > in copy_page.S and the main difference is copy_mc_page() add extable entry
> > to every load/store insn to support machine check safe. largely to keep the
> > patch simple. If needed those optimizations can be folded in.
> >
> > Add new extable type EX_TYPE_COPY_MC_PAGE which used in copy_mc_page().
> >
> > [1]https://lore.kernel.org/lkml/20221031201029.102123-1-tony.luck@xxxxxxxxx/
> >
> > Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
>
> This series needs rebasing onto a newer kernel. Some random comments
> below.

OK, very willing to do it :)

> > diff --git a/arch/arm64/lib/copy_mc_page.S b/arch/arm64/lib/copy_mc_page.S
> > new file mode 100644
> > index 000000000000..03d657a182f6
> > --- /dev/null
> > +++ b/arch/arm64/lib/copy_mc_page.S
> > @@ -0,0 +1,82 @@
> [...]
> > +SYM_FUNC_START(__pi_copy_mc_page)
> > +alternative_if ARM64_HAS_NO_HW_PREFETCH
> > +	// Prefetch three cache lines ahead.
> > +	prfm	pldl1strm, [x1, #128]
> > +	prfm	pldl1strm, [x1, #256]
> > +	prfm	pldl1strm, [x1, #384]
> > +alternative_else_nop_endif
> > +
> > +CPY_MC(9998f, ldp	x2, x3, [x1])
> > +CPY_MC(9998f, ldp	x4, x5, [x1, #16])
> > +CPY_MC(9998f, ldp	x6, x7, [x1, #32])
> > +CPY_MC(9998f, ldp	x8, x9, [x1, #48])
> > +CPY_MC(9998f, ldp	x10, x11, [x1, #64])
> > +CPY_MC(9998f, ldp	x12, x13, [x1, #80])
> > +CPY_MC(9998f, ldp	x14, x15, [x1, #96])
> > +CPY_MC(9998f, ldp	x16, x17, [x1, #112])
> [...]
> [...]
> > +9998:	ret
>
> What I don't understand, is there any error returned here or the bytes
> not copied? I can see its return value is never used in this series.
>
> Also, do we need to distinguish between fault on the source or the
> destination?

Oh, missing it, This should rerun bytes not copied.
will be fixed next version.

> > diff --git a/arch/arm64/lib/mte.S b/arch/arm64/lib/mte.S
> > index 5018ac03b6bf..bf4dd861c41c 100644
> > --- a/arch/arm64/lib/mte.S
> > +++ b/arch/arm64/lib/mte.S
> > @@ -80,6 +80,25 @@ SYM_FUNC_START(mte_copy_page_tags)
> >   	ret
> >   SYM_FUNC_END(mte_copy_page_tags)
> >   
> > +/*
> > + * Copy the tags from the source page to the destination one wiht machine check safe
> > + *   x0 - address of the destination page
> > + *   x1 - address of the source page
> > + */
> > +SYM_FUNC_START(mte_copy_mc_page_tags)
> > +	mov	x2, x0
> > +	mov	x3, x1
> > +	multitag_transfer_size x5, x6
> > +1:
> > +CPY_MC(2f, ldgm	x4, [x3])
> > +	stgm	x4, [x2]
> > +	add	x2, x2, x5
> > +	add	x3, x3, x5
> > +	tst	x2, #(PAGE_SIZE - 1)
> > +	b.ne	1b
> > +2:	ret
> > +SYM_FUNC_END(mte_copy_mc_page_tags)
>
> While the data copy above handles errors on both source and destination,
> here you skip the destination. Any reason?

Oh, my fault, miss the destination.


> > diff --git a/arch/arm64/mm/copypage.c b/arch/arm64/mm/copypage.c
> > index 8dd5a8fe64b4..005ee2a3cb4e 100644
> > --- a/arch/arm64/mm/copypage.c
> > +++ b/arch/arm64/mm/copypage.c
> [...]
> > +#ifdef CONFIG_ARCH_HAS_COPY_MC
> > +void copy_mc_highpage(struct page *to, struct page *from)
> > +{
> > +	void *kto = page_address(to);
> > +	void *kfrom = page_address(from);
> > +
> > +	copy_mc_page(kto, kfrom);
> > +	do_mte(to, from, kto, kfrom, true);
> > +}
> > +EXPORT_SYMBOL(copy_mc_highpage);
> > +
> > +int copy_mc_user_highpage(struct page *to, struct page *from,
> > +			unsigned long vaddr, struct vm_area_struct *vma)
> > +{
> > +	copy_mc_highpage(to, from);
> > +	flush_dcache_page(to);
> > +	return 0;
> > +}
>
> This one always returns 0. Does it actually catch any memory failures?

Yes, will be fixed next version.

> > +EXPORT_SYMBOL_GPL(copy_mc_user_highpage);
> > +#endif
> > diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> > index 28ec35e3d210..0fdab18f2f07 100644
> > --- a/arch/arm64/mm/extable.c
> > +++ b/arch/arm64/mm/extable.c
> > @@ -16,6 +16,13 @@ get_ex_fixup(const struct exception_table_entry *ex)
> >   	return ((unsigned long)&ex->fixup + ex->fixup);
> >   }
> >   
> > +static bool ex_handler_fixup(const struct exception_table_entry *ex,
> > +			     struct pt_regs *regs)
> > +{
> > +	regs->pc = get_ex_fixup(ex);
> > +	return true;
> > +}
>
> Should we prepare some error here like -EFAULT? That's done in
> ex_handler_uaccess_err_zero().

Yes, it should be done here and will be fixed next version.

Thank you for these great suggestions.
Tong.

>