Hi Prasad, (My comment inline) On Sat, Dec 18, 2010 at 5:24 PM, Rajat Sharma <fs.rajat@gmail.com> wrote:
I am not sure if memory what for your filesystem needs mapping, may be to support binary execution. Do you really need to support Writable or Shared memory mappings? If you are good with Read-only mappings or Private memory mapping (which does not modify data in page cache but modify a temporary COWed copy), the solution is simple - do not implement writepage method at all, there won't be any dirty data in page cache at any point of time, so no need to flush page cache, however you need to discard page cache range when you modify the file through f_op->write(), otherwise memory mapped files will read stale data.
Rajat
On Sat, Dec 18, 2010 at 6:51 AM, Prasad Joshi <prasadjoshi124@gmail.com> wrote:
Hello All,
I am working on a file system, that uses page cache only if a file is memory mapped. This is how the code is designed
write() { write the data onto the disk
if (file not memory mapped) return;
/* file is memory mapped */ while (till all data is not syced with page cache) { if (data at current file offset held in page) { update the page } } }
The code that updates a page is something like this page = find_lock_page(mapping, index); if (likely(!page)) { /* page not cached */ continue; }
if (mapping_writably_mapped(mapping)) flush_dcache_page(page);
pagefault_disable();
/* copy the data */ BUG_ON(!in_atomic()); page_buf = kmap_atomic(page, KM_USER0); if (__copy_from_user_inatomic(page_buf+offset, user_buf, size)) { kunmap(page); unlock_page(page); err = -EFAULT; goto out_error; } kunmap_atomic(page_buf, KM_USER0); pagefault_enable();
flush_dcache_page(page); mark_page_accessed(page);
SetPageUptodate(page); ClearPageError(page); unlock_page(page); page_cache_release(page);
If a fops->write() is called the data in page cache is already on to the disk. I don't want a page update from this function to trigger writepage() function.
The writepage() function should only be called if an user updates a memory mapped page.
Would calling page_clear_dirty() from the write() code be sufficient?
Are you sure that once you have cleared the dirty bit no other process mapping this file is performing any modifications? If yes, probably that should suffice.
I need to call flush_dcache_page() as a page should be coherent with other mappings. Does calling flush_dcache_page() result in call to writepage()?
I very faintly remember that flush_dcache_page would invoke the mmap operations which in turn would converge back into a filesystem specific writepage call. Hence, I would expect writepage to be called for dcache flush operation. Probably this might be helpful: http://lxr.linux.no/#linux+v2.6.36/Documentation/cachetlb.txt#L279 Also, had I been in your shoes, I would have actually put a printk like thing in my writepage operation to confirm if indeed my function was not getting called from any kernel specific threads (like pdflush etc). [Please take my suggestions with a pinch of salt as I have been out of touch with this topic since ages - and in case you find them incorrect, I would be very thankful in case you can correct me too] -- Shreyansh