Hi On Thu, May 15, 2014 at 12:35 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > The aspect which really worries me is this: the maintenance burden. > This approach would add some peculiar new code, introducing a rare > special case: which we might get right today, but will very easily > forget tomorrow when making some other changes to mm. If we compile > a list of danger areas in mm, this would surely belong on that list. I tried doing the page-replacement in the last 4 days, but honestly, it's far more complex than I thought. So if no-one more experienced with mm/ comes up with a simple implementation, I'll have to delay this for some more weeks. However, I still wonder why we try to fix this as part of this patchset. Using FUSE, a DIRECT-IO call can be delayed for an arbitrary amount of time. Same is true for network block-devices, NFS, iscsi, maybe loop-devices, ... This means, _any_ once mapped page can be written to after an arbitrary delay. This can break any feature that makes FS objects read-only (remounting read-only, setting S_IMMUTABLE, sealing, ..). Shouldn't we try to fix the _cause_ of this? Isn't there a simple way to lock/mark/.. affected vmas in get_user_pages(_fast)() and release them once done? We could increase i_mmap_writable on all affected address_space and decrease it on release. This would at least prevent sealing and could be check on other operations, too (like setting S_IMMUTABLE). This should be as easy as checking page_mapping(page) != NULL and then adjusting ->i_mmap_writable in get_writable_user_pages/put_writable_user_pages, right? Thanks David -- 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
