On Tue, May 14, 2024 at 07:35:15PM +0100, Matthew Wilcox wrote: > As encouraged by my lively audience / collaborators, here is the > typos-and-all result of the LSFMM session which just finished. > > Please don't respond to point out the typos; trust that I will fix them. May I bikeshed starting sentences on a new line for better diffability, then? Arbitrary incoherent comments follow: > ========== > Page flags > ========== > > Struct page includes a 'flags' word. Despite its name, various other > things are also stored in this word, including the page's node, zone, > section, and last_cpupid. See include/linux/page-flags-layout.h for > details. > > Locked > ====== > > This flag is per-folio. If you attempt to lock a page, you will lock > the entire folio. The folio lock is used for many purposes. In the page > cache, folios are locked before reads are started and unlocked once the > read has completed. The folio is also locked before writeback starts; > see the writeback flag for more detail. The truncation path takes the > folio lock, and folios are also locked while being inserted into page > tables in order to prevent races between truncation and page fault. If you drop the lock of a pagecache folio and relock it, you have to revalidate mapping and index of the head, right? > ... more detail here please ... > > Writeback > ========= > > Per-folio > This is kind of a lock > We downgrade to it having taken the lock flag > Rekeased after wrut4back completes, but lock flag may be released any time after writeback flag set. Deopends on filesystem whether needs to do more between more between...? /me doesn't understand the sentence. > We can wait for writeback to complete by waiting on this flag > Folio put to tail of LRU for faster reclaim > > Can prevent tearing write is filesystem needs stable folios If the filesystem or the block device (e.g. T10 PI/RAID5) need stable folios. > Truncate will wait for flag to clear > > > Referenced > ========== > > Per-folio flag. At least one page table entry has a accessed bit set for this folio > We set this during scan. Also set during buffered IO. Referenced first time, Accessed second time. > Used during reclaim to determine dispotition (activate, reclaim, etc) > > Uptodate > ======== > > Every byte of the folio contents is at least as new as the contents of disk > implicit write bbarrier > > > Dirty > ===== > Also set during buffered IO. Referenced first time, Accessed second time. > Used during reclaim to determine dispotition (activate, reclaim, etc) > At least one byte of the folio contents is newer than on disk and the writeback flag is not yet set > folios may be both dirty and not uptodate > lazyfree pages can drop the dirty bit > dirty flag clear for file folios when we start writeback > set dirty flag when removed from swapcache > if already dirty, folios can be mapped writable without notifying filesystem > complicated interfaces to set easy to get wrong > redirty_for_writeback > > > > LRU > === > > FOlio has been added to the LRU and is no longer in percpu folio_batch > > > Head > ==== > > This folio is a large folio. It is not set on order-0 folios. Is it not set on the non-head pages as well? > > Waiters > ======= > > Page has waiters, check its waitqueue. > Only used by core code. Don't touch. > > Active > ====== > > On the active LRU list. > Can be set in advance to tell kernel to put it on the right list > > Workingset > ========== > > Set on folios in pagecache once readahead pages actually accessed > Set on LRU pages that were activated have been deactivated, treat refault as thrashing > refault handler also sets it on folios that were hot before reclaimed > used by PSI computation > > Owner_Priv_1 > ============ > > Owner use. If pagecache, fs may use > > Used as Checked flag by many filesystems. > Used as SwapBacked flag by swap code. > > Arch_1 > ====== > > Many different uses depending on architecture. Oftern used as a "dcache clean" or, comfusingly as "dcache dirty". Check with your architecture. > > s390 uses it for basically everything. > > Historicalkly was used on a per page basis. Think we've eliminated all per-page uses now so should only be set on folios. > > Reserved > ======== > > > > Private > ======= > > If pagecache, has fs-private data > > Private_2 > ========= > > If pagecache, has fs aux data > > Mapped_to_disk > ============== > > Has blocks allocated on-disk > > Reclaim > ======= > > To be reclaimed asap > > Swap_backed > =========== > > Page is backed by RAM/swap > > Unevictable > =========== > > Page is "unevictable" > > Mlocked > ======= > > Page is vma mlocked > > Uncached > ======== > > Page has been mapped as uncached > > HWPoison > ======== > > hardware poisoned page. Don't touch I forgot the results of our expedition into why shmem.c checks PageHWPoison so much vs checking for EIO on the file/inode/etc. My hazy recollection is that poisoning a pagecache folio also sets AS_EIO too? If that's true, maybe we should mention that. --D > Young > ===== > > Idle > ==== > > Arch 2 > ====== > > Arch 3 > ====== > > Readahead > ========= > > Aliases with Reclaim as they are usually never set on the same folio, and > if they are the consequences are a minor performance loss. > > Anon_Exclusive > ============== > > Aliases with Mapped_To_Disk as that flag can never be set on anonymous folios. > > Isolated > ======== > > Shared with Reclaim flag, only valid for folios with LRU flag clear. > > Reported > ======== > > Only valid for buddy pages. Used to track pages that are reported > > Has_HWPoisoned > ============== > > Large_Rmappable > =============== > > >
