On 29/10/2024 02:50, Andres Freund wrote:
Hi,I just noticed that fsm_vacuum_page() modifies a buffer without even holding a shared lock. That quite obviously seems like a violation of the buffer locking protocol: /* * Try to reset the next slot pointer. This encourages the use of * low-numbered pages, increasing the chances that a later vacuum can * truncate the relation. We don't bother with a lock here, nor with * marking the page dirty if it wasn't already, since this is just a hint. */ if (BufferPrepareToSetHintBits(buf)) { ((FSMPage) PageGetContents(page))->fp_next_slot = 0; BufferFinishSetHintBits(buf); } In the commit (15c121b3ed7) adding the current freespace code, there wasn't even a comment remarking upon that oddity. 10 years later Tom added a comment, in 2b1759e2675f. I noticed this while adding a debug mode in which buffers are mprotected PROT_NONE/PROT_READ/PROT_READ|PROT_WRITE depending on the buffer's state. Is there any good reason to avoid a lock here? Compared to the cost of exclusively locking buffers during RecordAndGetPageWithFreeSpace() the cost of doing so during FreeSpaceMapVacuum*() seems small?
Agreed. This is a premature optimization, fsm_vacuum_page() should just take the lock.
Somewhat relatedly, but I don't think I understand why it's a good idea to reset fp_next_slot to 0 in fsm_vacuum_page(). At least doing so unconditionally.
Per the comment: "This encourages the use of low-numbered pages, increasing the chances that a later vacuum can truncate the relation".
Yes, the next GetPageWithFreeSpace() call will need to do a little more work to find the first page that actually has free space, if any. But that seems insignificant compared to vacuum.
When extending a relation, it seems we'll constantly reset the search back to the start of the range, even though we pretty much know that there's no space earlier in the relation - otherwise we'd not have extended.
That's a good point. Before commit a063baaced, relation extension used a separate UpdateFreeSpaceMap() function, which didn't reset fp_next_slot.
And when called from FreeSpaceMapVacuumRange() we'll reset fp_next_slot to somewhere that wasn't actually vacuumed, afaict?
Yeah. In the context of actual VACUUM rather than relation extension, that seems fine; the next GetPageWithFreeSpace() call will fix it up quickly.
-- Heikki Linnakangas Neon (https://neon.tech)
