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SubjectRe: [BUG] 2.6.1/MIPS - missing cache flushing when user program returns pages to kernel
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
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