page_cache_prev_miss(9)

💅 page_cache_prev_miss — Kernel API Manual 🎀

okay bestie, welcome to the Kernel API Manual entry for page_cache_prev_miss and trust me, this one is lowkey iconic once you get the vibe ✨

🌸 NAME 🌸

page_cache_prev_miss — find the previous gap in the page cache, fr fr 🫶

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✨ SYNOPSIS ✨

pgoff_t page_cache_prev_miss (struct address_space *mapping , pgoff_t
       index , unsigned long max_scan );

that's the whole signature, no cap 💖

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🦋 ARGUMENTS 🦋

• mapping — the mapping. yeah, that's literally it, it's the mapping bestie 💀
• index — the index. we love a self-explanatory name 👑
• max_scan — the maximum range to search, so like, how far back we're willing to go looking 🔥

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💜 DESCRIPTION 💜

so the way this works: it searches the range [max(index - max_scan + 1, 0), index] looking for the gap with the highest index. that's the whole mission, periodt 🌈

okay but here's the caveat and it's kind of a big one 🙈 — you may call this function under the rcu_read_lock. however, that does not atomically search a snapshot of the cache at a single point in time. it's giving "vibes, not guarantees" ngl.

for example (and this is the example from the docs, I'm not making it up): if a gap is created at index 10, and then subsequently a gap is created at index 5, page_cache_prev_miss covering both indices may return 5 if called under the rcu_read_lock. so yeah, the answer can be the older/lower one depending on timing 😭 that's just how it is under RCU.

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⭐ RETURN ⭐

• the index of the gap if found 💖
• otherwise an index outside the range specified — and in that case, index - return >= max_scan will be true. handy check, iykyk 🧚
• and in the rare case of wrap-around, ULONG_MAX will be returned 👀 that's the special one, don't forget it

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🍓 SEE ALSO 🍓

Kernel file ./mm/filemap.c, plus the whole friend group 💅:

• filemap_remove_folio(9)
• filemap_fdatawrite_range(9)
• filemap_flush_range(9)
• filemap_flush(9)
• filemap_range_has_page(9)
• filemap_fdatawait_range(9)
• filemap_fdatawait_range_keep_errors(9)
• file_fdatawait_range(9)
• filemap_fdatawait_keep_errors(9)
• filemap_write_and_wait_range(9)
• file_check_and_advance_wb_err(9)
• file_write_and_wait_range(9)
• replace_page_cache_folio(9)
• softleaf_entry_wait_on_locked(9)
• folio_put_wait_locked(9)
• folio_unlock(9)
• folio_end_read(9)
• folio_end_private_2(9)
• folio_wait_private_2(9)
• folio_wait_private_2_killable(9)
• folio_end_writeback_no_dropbehind(9)
• folio_end_writeback(9)
• __folio_lock(9)
• page_cache_next_miss(9)
• __filemap_get_folio_mpol(9)
• find_get_entries(9)
• find_lock_entries(9)
• filemap_get_folios(9)
• filemap_get_folios_contig(9)
• filemap_get_folios_tag(9)
• filemap_get_folios_dirty(9)
• filemap_read(9)
• generic_file_read_iter(9)
• filemap_splice_read(9)
• mapping_seek_hole_data(9)
• filemap_fault(9)
• read_cache_folio(9)
• mapping_read_folio_gfp(9)
• read_cache_page_gfp(9)
• __generic_file_write_iter(9)
• generic_file_write_iter(9)
• filemap_release_folio(9)
• filemap_invalidate_inode(9)
• filemap_cachestat(9)

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and that's the page bestie 🫶 source location is ./mm, dated October 2026, filed under page_cache_prev_miss(9) — go forth and find those gaps, you absolute main character 💖✨🎀