Readahead: the documentation I wanted to read

Ah, I see that in my inbox now ... I read the third email in the chain (the first two went only to xfs?), but didn't read the fourth and fifth. The usual too-much-email problem.

Anyway, I think recognising this special case probably isn't the right solution. Backup is always tricky, and your proposal would fix one-large-file but do nothing for many-small-files.

I suspect the right way to go is to recognise that the page cache is large and has many easily-reclaimable pages, and shrink only the page cache. ie the problem is that backup is exerting general memory pressure when we'd really like it to only exert pressure only on the page cache. Or rather, we'd like the page cache to initially exert pressure only on the page cache. The dcache should initially exert pressure only on the dcache. Etc. If a cache can't reclaim enough memory easily, then it should pressure other caches to shrink.