Messages in this thread | | | Date | Mon, 06 Apr 2009 16:23:18 -0400 | From | Masami Hiramatsu <> | Subject | Re: [BUGFIX][PATCH -tip] x86: fix text_poke to handle highmem pages |
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Mathieu Desnoyers wrote: > * Masami Hiramatsu (mhiramat@redhat.com) wrote: >> Mathieu Desnoyers wrote: >>> * Masami Hiramatsu (mhiramat@redhat.com) wrote: >>>> Fix a bug in text_poke to handle highmem pages, because module >>>> text pages are possible to be highmem pages on x86-32. >>>> In that case, since fixmap can't handle those pages, text_poke >>>> uses kmap_atomic. >>>> >>> Hrm, can you remind me what would be the downside of using kmap_atomic >>> in every scenarios (highmem and non-highmem) then ? >> kmap_atomic can handle only highmem pages. If you passes lowmem pages, >> it returns just original vaddr of it. (because kmap is only for >> highmem support) >> > > OK, and if we are doing a second kmap_atomic() of a module text page > which is already mapped, does the second kmap_atomic return the vaddr of > the page originally mapped or is it creating a second mapping ?
Good point, kmap_atomic creates a second mapping if the page is highmem.
in arch/x86/mm/highmem_32.c:
void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot) { ... if (!PageHighMem(page)) return page_address(page); ... }
> Because if we ever decide to enforce read-only page mapping for module > text pages, touching highmem pages too, we will run into real trouble if > those happen to be the same page.
So, as long as the page is in highmem, we can create a second mapping by using kmap_atomic.
Thank you,
> > Mathieu > >>> I would try to avoid "special cases" as much as possible, because they >>> just make problems harder to reproduce. >> Actually, this bug is a special case because it happens only on PAE kernel. >> >>> The idea would be to either add fixmap highmem support, or to simply use >>> kmap_atomic() for all cases until we add fixmap highmem support. >>> >> >> Thank you, >> >> -- >> Masami Hiramatsu >> >> Software Engineer >> Hitachi Computer Products (America) Inc. >> Software Solutions Division >> >> e-mail: mhiramat@redhat.com >> >
-- Masami Hiramatsu
Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division
e-mail: mhiramat@redhat.com
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