2.6.13-rc3: cache flush missing from somewhere

Russell King
2005-07-29T15:14:22Z

Hi,

I've been trying for the last 4 or 5 days to get the kernel stable on
an ARM SMP platform. This platform has harvard PIPT caches with no
aliasing issues inside the separate I/D caches, except for the lack
of snooping between the I and D cache. The caches are in write allocate
mode.

This means we need to ensure that the I/D coherency is handled, and
we do this via flush_dcache_page(). We actually do this lazily using
the Sparc64 method, so __flush_dcache_page() actually does the cache
operations.

My current patch to get this working is below. The only thing which
really seems to fix the issue is the __flush_dcache_page call in
read_pages() - if I remove this, I get spurious segfaults and illegal
instruction faults.

If I make flush_dcache_page() non-lazy, this also fixes it, but this
is not desirable. The problem also goes away if I disable the write
allocate cache mode.

If I call __flush_dcache_page() from update_mmu_cache() (iow, always
ensure that we have I/D coherency when the page is mapped into user
space) the effect is the same - I see random faults.

This is using cramfs as the filesystem, which does call flush_dcache_page()
on pages returned via its readpages implementation.

Unfortunately, I've only recently obtained this hardware, but I know
a previous kernel (2.6.7-based) works fine on it (already supplied by
others.) However, there's a massive delta from mainline for this
which makes it totally impractical to try other mainline kernels.

To me, it feels like there's a path which results in pages mapped into
user space without update_mmu_cache() being called, but I'm unable to
find it. Ideas?

diff -up -x BitKeeper -x ChangeSet -x SCCS -x _xlk -x '*.orig' -x '*.rej' -r orig/mm/filemap.c linux/mm/filemap.c
--- orig/mm/filemap.c Wed Jun 29 15:52:51 2005
+++ linux/mm/filemap.c Fri Jul 29 15:32:40 2005
@@ -849,6 +849,8 @@ readpage:
unlock_page(page);
}

+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
/*
* i_size must be checked after we have done ->readpage.
*
@@ -1158,6 +1160,8 @@ static int fastcall page_cache_read(stru
error = add_to_page_cache_lru(page, mapping, offset, GFP_KERNEL);
if (!error) {
error = mapping->a_ops->readpage(file, page);
+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
page_cache_release(page);
return error;
}
@@ -1254,7 +1258,9 @@ retry_find:
page = find_get_page(mapping, pgoff);
if (!page)
goto no_cached_page;
+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));
}
+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));

if (!did_readaround)
ra->mmap_hit++;
@@ -1267,6 +1273,8 @@ retry_find:
goto page_not_uptodate;

success:
+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
/*
* Found the page and have a reference on it.
*/
@@ -1402,6 +1410,8 @@ retry_find:
}

success:
+if (page->mapping && !mapping_mapped(page->mapping)) BUG_ON(!test_bit(PG_dcache_dirty, &page->flags));
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
/*
* Found the page and have a reference on it.
*/
@@ -1508,6 +1518,7 @@ repeat:
if (!page && !nonblock)
return -ENOMEM;
if (page) {
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
err = install_page(mm, vma, addr, page, prot);
if (err) {
page_cache_release(page);
diff -up -x BitKeeper -x ChangeSet -x SCCS -x _xlk -x '*.orig' -x '*.rej' -r orig/mm/memory.c linux/mm/memory.c
--- orig/mm/memory.c Wed Jun 29 15:52:51 2005
+++ linux/mm/memory.c Fri Jul 29 15:41:11 2005
@@ -1821,6 +1821,7 @@ do_no_page(struct mm_struct *mm, struct
retry:
cond_resched();
new_page = vma->vm_ops->nopage(vma, address & PAGE_MASK, &ret);
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(new_page->mapping, new_page); }
/*
* No smp_rmb is needed here as long as there's a full
* spin_lock/unlock sequence inside the ->nopage callback
diff -up -x BitKeeper -x ChangeSet -x SCCS -x _xlk -x '*.orig' -x '*.rej' -r orig/mm/readahead.c linux/mm/readahead.c
--- orig/mm/readahead.c Mon Apr 4 22:54:55 2005
+++ linux/mm/readahead.c Fri Jul 29 15:57:18 2005
@@ -172,6 +172,7 @@ static int read_pages(struct address_spa
if (!add_to_page_cache(page, mapping,
page->index, GFP_KERNEL)) {
mapping->a_ops->readpage(filp, page);
+{ void __flush_dcache_page(struct address_space *mapping, struct page *page); __flush_dcache_page(page->mapping, page); }
if (!pagevec_add(&lru_pvec, page))
__pagevec_lru_add(&lru_pvec);
} else {

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core