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Re: [BUG] 2.6.1/MIPS - missing cache flushing when user program returns

To: "David S. Miller" <davem@redhat.com>
Subject: Re: [BUG] 2.6.1/MIPS - missing cache flushing when user program returns pages to kernel
From: Jun Sun <jsun@mvista.com>
Date: Thu, 15 Jan 2004 10:03:30 -0800
Cc: linux-mips@linux-mips.org, jsun@mvista.com
In-reply-to: <20040114222316.25276f12.davem@redhat.com>; from davem@redhat.com on Wed, Jan 14, 2004 at 10:23:16PM -0800
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References: <20040114163920.E13471@mvista.com> <20040114171252.4d873c51.akpm@osdl.org> <20040114172946.03e54706.akpm@osdl.org> <20040114174012.H13471@mvista.com> <20040114222316.25276f12.davem@redhat.com>
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Looks the right fix is to add cache flush to tlb_start_vma().
See the patch attached.  Unless someone objects, I will check it
later.

BTW, I really don't like the function naming of tlb_start_vma()
and tbl_end_vma(). :)

Jun

On Wed, Jan 14, 2004 at 10:23:16PM -0800, David S. Miller wrote:
> On Wed, 14 Jan 2004 17:40:12 -0800
> Jun Sun <jsun@mvista.com> wrote:
> 
> > Looking at my tree (which is from linux-mips.org), it appears
> > arm, sparc, sparc64, and sh have tlb_start_vma() defined to call
> > cache flushing.
> 
> Correct, in fact every platform where cache flushing matters
> at all (ie. where flush_cache_*() routines actually need to
> flush a cpu cache), they should have tlb_start_vma() do such
> a flush.
> 
> > What exactly does tlb_start_vma()/tlb_end_vma() mean?  There is
> > only one invocation instance, which is significant enough to infer
> > the meaning.  :)
> 
> When the kernel unmaps a mmap region of a process (either for the
> sake of munmap() or tearing down all mapping during exit()) tlb_start_vma()
> is called, the page table mappings in the region are torn down one by
> one, then a tlb_end_vma() call is made.
> 
> At the top level, ie. whoever invokes unmap_page_range(), there will
> be a tlb_gather_mmu() call.
> 
> In order to properly optimize the cache flushes, most platforms do the
> following:
> 
> 1) The tlb->fullmm boolean keeps trap of whether this is just a munmap()
>    unmapping operation (if zero) or a full address space teardown
>    (if non-zero).
> 
> 2) In the full address space teardown case, and thus tlb->fullmm is
>    non-zero, the top level will do the explict flush_cache_mm()
>    (see mm/mmap.c:exit_mmap()), therefore the tlb_start_vma()
>    implementation need not do the flush, otherwise it does.
> 
>    This is why sparc64 and friends implement it like this:
> 
> #define tlb_start_vma(tlb, vma) \
> do {    if (!(tlb)->fullmm)     \
>                 flush_cache_range(vma, vma->vm_start, vma->vm_end); \
> } while (0)
> 
> Hope this clears things up.
> 
> Someone should probably take what I just wrote, expand and organize it,
> then add such content to Documentation/cachetlb.txt
> 

Attachment: real-fix.patch
Description: Text document

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