On 09/10/2014 10:25 AM, Boaz Harrosh wrote: > Yes the block_allocator of the pmem-FS always holds the final REF on this > page, as long as there is valid data on this block. Even cross boots, the > mount code re-initializes references. The only internal state that frees > these blocks is truncate, which only then return these pages to the block > allocator, all this is common practice in filesystems so the page-ref on > these blocks only ever drops to zero after they loose all visibility. And > yes the block allocator uses a special code to drop the count to zero > not using put_page(). OK, so what happens when a page is truncated out of a file and this "last" block reference is dropped while a get_user_pages() still has a reference? > On 09/10/2014 07:10 PM, Dave Hansen wrote: >> Does the fs support mmap()? >> > No! > > Yes the FS supports mmap, but through the DAX patchset. Please see > Matthew's DAX patchset how he implements mmap without using pages > at all, direct PFN to virtual_addr. So these pages do not get exposed > to the top of the FS. > > My FS uses his technics exactly only when it wants to spill over to > slower device it will use these pages copy-less. >From my perspective, DAX is complicated, but it is necessary because we don't have a 'struct page'. You're saying that even if we pay the cost of a 'struct page' for the memory, we still don't get the benefit of having it like getting rid of this DAX stuff? Also, about not having a zone for these pages. Do you intend to support 32-bit systems? If so, I believe you will require the kmap() family of functions to map the pages in order to copy data in and out. kmap() currently requires knowing the zone of the page. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
