Problem

The NOT EXISTS clause in GET_FULL_PAGE_LSN (ext/page_log/palite/palite.cpp ~line 1407) incorrectly excludes valid full pages from the backlink lookup when a discarded delta with matching base_lsn exists. A discarded delta means the delta is superseded, not its base full page. This causes verify_chain to fail with "Full page backlink_lsn mismatch ... expected: 0", returning EINVAL and panicking the eviction thread.

Evidence

Page log state for table_id=57, page_id=1965:

lsn=546143, backlink=463197, base=0,      delta=0, discarded=0  <\- new full page \(being written\)
lsn=545145, backlink=463197, base=463197,  delta=1, discarded=1  <\- discarded delta
lsn=463197, backlink=461766, base=0,       delta=0, discarded=0  <\- valid previous full page

The new full page (546143) has backlink_lsn=463197. The full page at 463197 is valid. But the NOT EXISTS clause finds the discarded delta (545145) with base_lsn=463197 and excludes the full page. get_prev_full_page_lsn returns 0, verification fails.

Fix

Remove the NOT EXISTS clause from GET_FULL_PAGE_LSN:

SELECT p1.lsn
FROM pages AS p1, full\_page AS fp
WHERE p1.table\_id = ?1 AND p1.page\_id = ?2
    AND p1.lsn = fp.backlink\_lsn AND p1.delta = 0;

Impact

• Affects eviction, checkpoint, and cursor operations on disaggregated storage
• Not specific to two-phase eviction
• CI: https://evergreen.mongodb.com/version/69d757b6dff295000716cd66