On Mon, 2024-01-29 at 04:32 +0000, Matthew Wilcox wrote: > Our documentation of the current page flags is ... not great. I think > I can improve it for the page cache side of things; I understand the > meanings of locked, writeback, uptodate, dirty, head, waiters, slab, > mlocked, mappedtodisk, error, hwpoison, readahead, anon_exclusive, > has_hwpoisoned, hugetlb and large_remappable. > > Where I'm a lot more shaky is the meaning of the more "real MM" flags, > like active, referenced, lru, workingset, reserved, reclaim, swapbacked, > unevictable, young, idle, swapcache, isolated, and reported. > > Perhaps we could have an MM session where we try to explain slowly and > carefully to each other what all these flags actually mean, talk about > what combinations of them make sense, how we might eliminate some of > them to make more space in the flags word, and what all this looks like > in a memdesc world. > > And maybe we can get some documentation written about it! Not trying > to nerd snipe Jon into attending this session, but if he did ... > > [thanks to Amir for reminding me that I meant to propose this topic] > On the "Reclaiming" part of this thread, we might consider this: Optimizing Page Flags: Reclaiming Bits in page->flags via folio->lru Limited bit space in the Linux kernel's page->flags field, especially on 32-bit architectures, is a source of challenge [1]. This proposal aims to free up bits by relocating flags like PG_active and PG_unevictable to the lower bits of folio->lru as they are always unset. It helps with encoding zone, numa node, and sparsemem section [2]. Proposed Process: Candidate Evaluation: Assess flags for relocation suitability based on usage, dependencies, and functional impact. Impact Assessment: Evaluate the impact on kernel code to ensure correct behavior and compatibility. Relocation Implementation: Modify code to read/write flags from folio->lru and adjust related macros/functions. Thoroughly test changes. [1] https://lwn.net/Articles/335768/ [2] https://blogs.oracle.com/linux/post/struct-page-the-linux-physical-page-frame-data-structure
