Re: msync() seems not to clean the data cache

[Matthew Wilcox <willy@infradead.org>]

On Fri, Apr 25, 2025 at 12:17:37PM -0700, John Reiser wrote:
> The system call write() cleans the hardware data cache (writes any dirty
> values from the data cache into RAM memory) before passing to the VFS
> the region to be written.  The system call msync() should do likewise.
> Currently msync() does not clean the hardware data cache, as seen on
> PowerPC, PowerPC64, and arm64; and probably any CPU that does not
> have a Write-through cache.  (x86 and x86_64 do have write-through.)

I think you're right; we don't flush before writeback.  Does this
fix your problem?

+++ b/mm/page-writeback.c
@@ -2432,6 +2432,7 @@ static bool folio_prepare_writeback(struct address_space *mapping,
        if (!folio_clear_dirty_for_io(folio))
                return false;

+       flush_dcache_folio(folio);
        return true;
 }


By the way, x86 does not have writethrough caches.  Instead it has DMA
coherent caches, so modified cachelines in the cache will take
precedence over the contents of memory.