Countably generated flat modules are quite flat

We prove that if $R$ is a commutative Noetherian ring, then every countably generated flat $R$-module is quite flat, i.e., a direct summand of a transfinite extension of localizations of $R$ in countable multiplicative subsets. We also show that if the spectrum of $R$ is of cardinality less than $\kappa$, where $\kappa$ is an uncountable regular cardinal, then every flat $R$-module is a transfinite extension of flat modules with less than $\kappa$ generators. This provides an alternative proof of the fact that over a commutative Noetherian ring with countable spectrum, all flat modules are quite flat. More generally, we say that a commutative ring is CFQ if every countably presented flat $R$-module is quite flat. We show that all von Neumann regular rings and all $S$-almost perfect rings are CFQ. A zero-dimensional local ring is CFQ if and only if it is perfect. A domain is CFQ if and only if all its proper quotient rings are CFQ. A valuation domain is CFQ if and only if it is strongly discrete.Comment: 17 pages. v2: a new author joined, Remark 3.7 inserted, the paper grew in size by a factor of more than two with a treatment of non-Noetherian rings added in new Sections 4 and 5. v.3: small improvements, abstract updated, the former Example 4.21 moved to Section 5 and details added in Section 5 (Theorem 5.1 inserted); v.4: small things corrected, a reference updated -- this is the final versio