On Wed, Feb 08, 2023 at 12:33:37PM -0800, Yosry Ahmed wrote: > On Wed, Feb 8, 2023 at 9:45 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Wed, Feb 08, 2023 at 08:01:01AM -0800, Luis Chamberlain wrote: > > > On Tue, Feb 07, 2023 at 04:01:51AM +0000, Matthew Wilcox wrote: > > > > On Mon, Feb 06, 2023 at 06:52:59PM -0800, Luis Chamberlain wrote: > > > > > @@ -1334,11 +1336,15 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) > > > > > struct shmem_inode_info *info; > > > > > struct address_space *mapping = folio->mapping; > > > > > struct inode *inode = mapping->host; > > > > > + struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb); > > > > > swp_entry_t swap; > > > > > pgoff_t index; > > > > > > > > > > BUG_ON(!folio_test_locked(folio)); > > > > > > > > > > + if (wbc->for_reclaim && unlikely(sbinfo->noswap)) > > > > > + return AOP_WRITEPAGE_ACTIVATE; > > > > > > > > Not sure this is the best way to handle this. We'll still incur the > > > > oevrhead of tracking shmem pages on the LRU, only to fail to write them > > > > out when the VM thinks we should get rid of them. We'd be better off > > > > not putting them on the LRU in the first place. > > > > > > Ah, makes sense, so in effect then if we do that then on reclaim > > > we should be able to even WARN_ON(sbinfo->noswap) assuming we did > > > everthing right. > > > > > > Hrm, we have invalidate_mapping_pages(mapping, 0, -1) but that seems a bit > > > too late how about d_mark_dontcache() on shmem_get_inode() instead? > > > > I was thinking that the two calls to folio_add_lru() in mm/shmem.c > > should be conditional on sbinfo->noswap. > > > > Wouldn't this cause the folio to not show up in any lru lists, even > the unevictable one, which may be a strange discrepancy? > > Perhaps we can do something like shmem_lock(), which calls > mapping_set_unevictable(), which will make folio_evictable() return > true and the LRUs code will take care of the rest? If shmem_lock() should take care of that is that because writepages() should not happen or because we have that info->flags & VM_LOCKED stop gap on writepages()? If the earlier then shouldn't we WARN_ON_ONCE() if writepages() is called on info->flags & VM_LOCKED? While I see the value in mapping_set_unevictable() I am not sure I see the point in using shmem_lock(). I don't see why we should constrain noswap tmpfs option to RLIMIT_MEMLOCK Please correct me if I'm wrong but the limit seem to be designed for files / IPC / unprivileged perf limits. On the contrary, we'd bump the count for each new inode. Using shmem_lock() would also complicate the inode allocation on shmem as we'd have to unwind on failure from the user_shm_lock(). It would also beg the question of when to capture a ucount for an inode, should we just share one for the superblock at shmem_fill_super() or do we really need to capture it at every single inode creation? In theory we could end up with different limits. So why not just use mapping_set_unevictable() alone for this use case? Luis
